A vendor collection is a list of items that can be sold by a vendor.
To create a vendor collection, find (or create) the GameName/Resources/GameName/VendorCollection folder in the project tab and right click. Choose Create > AnyRPG > VendorCollection.
Vendor Collections have a single property, Vendor Items, which is a list with each element having the following fields.
Item Quality Name
The name of an Item Quality. If this field is not empty, the Item purchased will be of this quality instead of its default quality.