Yes, invested items would show the sigil of the magus who created them.
Unless you stamp it with someone elses name, and include a linked trigger Shroud Magic linked to all the effects in the device.. ..
Then Perdo Vim any minor lingering traces to you before you get your Venditore to sell it, while disguised as some other magi's Venditore >.>
.. .. who would do such a malicious thing?! <.<
Oh that's wicked! I like it!
I was really surpised when no one said "Well first you gotta take their little shorts off..." You are all so much more mature than I am.
As far as how it turns out, our group of 3 mages and one companion has been out for only a couple of hours and theres already a lynch mob after us. Yeah, were reeeeeeaaaal subtle.
(Oh, just a reminder, my story guide doesn't want you to advise me in any way. Thanks!)
We were looking for a child that was kidnapped by faeries and one of our party who is blessed with blatant gift tried to scare information out of a witness. This was played out as one of the players standing over the story guide screaming at him while the story guide shreiked in a womans voice and faked a faint. The other two players were arguing and missed the whole thing. I was laughing too hard to tell them to pay attention.
So the woman gets away and a short time later, some people show up with torches and pitch forks....
In the interests of not-advising you , last time i played , our troupe was involved in a similar story.
A child had been kidnapped by the fae.
However , as my character was the one with the Gentle Gift
and a modicum of social skills , he did the questioning.
The short-sighted fellow Ex Misc , our enfeebled Jerbiton
and the sword-wielding Blatantly Gifted Tytalus did not start a pitchfork & torchfest. 
It sounds like you guys did better than we did.
Our companion, a knight (or so he says) saw them beginning to form the lynch mob. He snuck quietly away carrying his gigantic sword with him. We do have one of our party with gentle gift, but the lynch mob was being led by an angry priest and he was unable to put any kind of charm spells on the guy. The other two mages, including me, have blatant gift and I could either be a really large bird who is a symbolic omen of death, of a skinny little girl with no clothes on. I stayed away from the people since I could only make matters worse.
Sooo... How bad is it to make a promise to a faerie?
You DID that!?!?!? Holy shite, you are in deep trouble BellaDonna 
In fact, it depends on what did you exactly promnise, but if your SG is half the bastard that most SG are when playing the fae, expect something at least.... entertaining... coming your way.
Xavi
OH NO is it really that bad? I promised I would make sure her family pays attention to her or I would play with her myself. Also I drank faerie wine and spent a couple rounds face down in the dirt.
<>
Ask yourself this: "What WOULDN'T my storyguide do?" Fortunately, the SG has to keep the story fun...
-Verticius
p.s. Who had to negotiate with the UNseelie Fae as a new player...
Fae can have a very literal interpretation of a promise, and they have no problem with sophism- twisting words in not-quite rational ways, or taking common expressions quite literally, etc.
You promised her family would pay attention to her? Well, I'd say that is:
- forever
- "attention" until the faerie views it as "enough" (read 24/7?)
- "making sure" sounds like an absolute - if you break your promise (in the eyes of the faerie), even for a moment, no matter what excuse, lotsa luck.
- gonna become even tougher if anything ever happens to the family. (How do the dead "pay attention" to a faerie? Your maga promised that she'd make sure they would...!!!)
In short, you're screwed.
Better start thinking of ways that you can make that promise come true, working on the family with magic or such. Think outside the box- most such promises can be worked around if you turn the tables somehow. But that depends on the exact words your maga used.
The other mages were trying to argue with the faerie king. They said the kid's family wanted her. He said they didn't. I said "Who cares. I'll take her." I only said it because it seemed to solve the problem. I wasn't trying to gain a dependent.
Oh dear...What are some "keep a small kid occupied" spells?
...
... My GOD... my GM-sense is Tingling.
You know if I was the GM I'd have that very non-comforting grin right now. ... Unless you manage to gain the fairies family's attention for her? You are not just shafted, you will be her toy for the rest of your life.
I am all too aware of that grin.
Hey, How much involvement with faeries do I need in order to use faerie magic? Does that require a warping point? (I didn't screw up THAT bad.)
Oh yes, you did. With faeries, the danger is in the details. Just think about it positively: you are gonna have loads of fun there. Or die trying.
Xavi features the patented Storyguide Grin (TM) in his face as well
Cheers,
Xavi
Apparently, you now have the answer to that question. 
One solution would be to get the fae to whom you made the promise to owe you a favour, and see if you can't get released from your deal.
Do keep us posted on how this all works out, won't you? 8)
I will post it here.
Don't feel too bad, our apprentice mage in the group during her gauntlet mangaged to: call up the Wild Hunt, give hair to another fey, and I keep thinking there was another thing. Possibly there was but it was a good action...my half-fae was likely too busy running from the Wild Hunt to notice it.
My mage also drank fey wine once and ate fey food another time. That is the trouble with a clash of cultures in his harming a guest you feed is just not done, nor is it polite to turn down an offer of food especially if the host is eatiing it.
But promises to fey are dangerous, eating or drinking fey food is dangerous, not knowing the rules is suicidal (well so my half fae would say anyways)...vague promises however are extremely dangerous. But on the other hand if you can get out of the fae regio and can manage a season or two of preperation time you likely can deal with the fae. Depending on exactly how powerful it is...and if you can't alone then that is why you have sodales.
That was why I did it. I didn't want to be rude.
So now I haveto keep an eye on this little girl... Well I have been looking for someone who will help me learn my muto corpus spells... hehe
hmmm...just keep in mind the "little girl" could be far older than your wizard (is likely to be) and may be a "little girl" due to some rather complex and totally bewildering rules to a "game" that is currently being played out in the court.
My half-fae's advice would be to first learn what the game is, then what the rules are, then plan to survive them. Also the whole "Seelie" vrs "Unseelie" question would be wise to answer. Not mind you that Seelie translates necessarily into good, while Unseelie generally means "out to get you" though not necessarily evil.
But question everything including the fact this is a "little girl" you are dealing with.
All this largely depends on how your SG views and plays the fae. We have many encounters with them, being in wales and they are right terrifying for the most part.