Ah. Well then: to the next issue: no such market exists for gifted children: no Gernicius to validate the bill of sale, no confirmation of vis transfers, no registry to indicate who has valid claim, and so on. In other words, there is no Tribunal that has confirmed that such a sale is valid, and no laws or anything regulating such sale. Merely claiming "I have a contract!" is not enough. I can put such a claim on a breath of air, or a rock, or a grog, or anything in existence. The fact that it is an apprentice is irrelevant. You're just putting a private price tag on it with someone else, and paying in vis.
If you go forth and set up such a legal infrastructure BEFOREHAND, and get agreement that such a thing is a good idea? Sure. I think both Maine75Man, Jonathan.Link, and myself agree that getting buyoff from all interested parties at Tribunal would be a fine thing. This was probably how the Thebean system was set up. Which would be an interesting contrast, to see how it plays out. But if you get that sort of buy-off, there's no need to use the whole "I have a vis contract" reason. You've already got buy-off from the relevant authorities, which would give you legal coverage anyway as a Procurement Agent. The vis thing is irrelevant, and it doesn't hold water legally anyway.
Your comment is that you "have a contract". OK. I have a contract for the air you're about to breathe. Fully written up on fancy parchment last month, for the delivery of all the air in this room to one of my sodales, for the price of a rook of vis. Signed and witnessed by a magus in good standing. The breath you just took makes it impossible for me to deliver on my contract.
Do you feel obliged to pay that debt? Probably not. Your defense would be "I never agreed to be a part of that private contract for an activity that is freely available to all, and there is no Tribunal-wide ruling that forces me to abide by it."
To which I respond, "Yes, but we've been trading this air back and forth for a rook of vis every month for the past five years! You can see it in the covenant ledger! fully signed and certified that I paid for all this air with a rook of vis last month. And my sodales paid me a rook for the air the month before that, and so on and so forth, back and forth. You interrupted our air-vis contract, so now you must pay the wergild of a rook."
It's a silly example, yes - but it illustrates the problem of placing enforceable private contracts on arbitrary public activities that 3rd parties are allowed, by law, to stumble into.
Like the hot-crossed buns, or the ability to walk through a door, or the air of a Covenant - from the perspective of the relevant law: (Covenent law, general custom, or the Code itself, depending on which example I'm using), the property was not yours to make a claim on. And once you are able to start assigning arbitrary vis values to arbitrary actions, regardless of who is involved or their views on the subject, then the whole legal concept of "wergild as legal concept" falls apart. Those with sufficient funds can make "vis contracts" for arbitrary activities, announce them, and then force those involved to do their bidding, or else pay the price.
So: in conclusion - I don't have a problem with the idea of an Apprentice Agent, certified by the Tribunal, to find and train apprentices for a year, and then sell that service to a magi - I would imagine that some Tribunals have Redcaps doing that already. I do have a problem with someone trying to use a private vis contract to claim forfeit immunity. The law doesn't work like that - if you can get the Tribunal to set up a public "cost basis" that ALSO laid out claims of protection for the person being the Agent? Sure. But it's not the vis cost that's granting the protection: it's the Tribunal ruling that explicitly states "we're interpreting this to be part of the permissible activities of finding and training an Apprentice".