I don't see any problem with a gift shop near an exhibit or placed so folks have to enter or exit through one. The operation of a shop is a legit thing for a museum, after all. Some places even place the shop so it can be visited with out paying the admission price for the museum. However, It is not a good practice to mix gift shop items with collections. It seems to me there should be a clear dividing line between the two. Properly placed a gift shop can supply an extra pair or two of eyes to keep watch on some part of the exhibit space.