Wouldn't verification for almost anything beyond the extremely basic need specialized people? I don't think it's reasonable to train every Gamestop employee to be able to verify everything from fancy Swiss watches to brand name clothing to playing cards. At least if you do it in a central location you can have actual experts do the verification.
The verification I'd be looking for immediately in-store would be "is this [product] or is this a brick?" - that once happened to me, with (for all I see sellers complain about excessively buyer-friendly policies) no recourse or compensation. If there's a more-complicated verification needed, the buyer could request that it be held onto until that's performed (elsewhere, if necessary). Verifiable chain of custody protects both buyers and sellers.
Not that a dodgy Gamestop merger is necessary for this, but I'd be (much) more likely to both buy and sell on eBay if it existed.
Yup, you save a few dollars per verification on shipping in exchange for spending tens of thousands on training each employee to verify items. Mind you these are retail employees so turnover is going to be high. Also since they need to be able to verify everything they won't have time to be in store since they'll constantly need to be studying new products. That or you just have them work off checklists for each product in which case the value of the verification isn't great since counterfeiters will just buy those check lists and manufacture to them.
The end result is much much higher cost per verification, and a less trusted quality of verification. But you save a few bucks on shipping.