If I'm looking for X, I'd much rather go to an online store, where I have access to several listings of X at varying prices and condition, vs. make a trip to a physical store, see if they have X, and hope that the condition and price matches my expectations.
Maybe if I'm not looking for X, and just want to browse a bunch of stuff (e.g. yardsale/flea market style), then a physical store could make sense.
But, to me, having this middle-man physical presence was already a problem, and eBay solved this.
I just don't feel like eBay needs GameStop.
Now, whether GameStop needs eBay is a different story. But GameStop is in trouble for two reasons:
1. Video games -- and therefore video game sales -- are moving to digital.
2. Physical stores are becoming a thing of the past for retail transactions.
eBay doesn't need GameStop's troubles.