Lay of the land, size up the competition, magical knives or armor- presumably they will cost vis which Tantalus would need to approve but he can at least look. He also wants to find out about any non-Hermetic magics, especially those that can be bought with silver...
The vendors are set up outslde the covenant walls to the north in a haphazard manner. By and large, similar vendors are near each other, but it doesn't seem to be the result of any planning.
There are a handful of vendors selling magical wares (including armour and weaponry), and some taking commissions from wizards for their masters to negotiate with buyers. For example, Lucius wishes to have Rubeus craft an item that will allow him to create a Wizard's Mount, so he arranges to have his people talk with their people to make a deal.
The only non-Hermetic magic Arthur can find for sale is from some Gruagachan from MacGruagach.