The flea markets are squeezed dry these days and people figured out their stuff is worth something. Also, they often check prices on ebay.
Here is a depiction of the evolution of how a pile of 25 games goes cycled back in the marker.
Step 1: Lady clears the basement and brings a stack of 25 Gamecube games to Value village for donation
Step 2: Resident Value village Squatter buys all the games for 1$ each, has a member card, only costs 18$
Step 3: Value village Squatter rents a table at the flea market and sells the game 5$ each.
Step 4: A flea market hound finds the games and deals the whole lot for 50$, he knows a few of the games are valuable.
Step 5: He goes to a video game reseller to sell the games for more than he paid. He is not even a gamer.
Step 6: Reseller sells all the games individually at full market price.
And the wheel turns
I go early at the flea market every weekend and I see exactly this happen. The same lady is there every week selling games she get at value village. The same dudes hunting for games are there too.
Next I see them bringing boxes of stuff at the video game store. It is a well oiled machine.