You don't know how much they sold. This could be a fence operation that just grew large.

Thiefs don't find a buyer everytime they steal something, they go to their "fence" who resells it. Say something is worth $200 used, the fence will pay like half or a third of the value in cash right away, every time. The thief walks away with $80 they can go spend on drugs, and the fence eventually makes $120. Reselling takes time though, and your thiefs keep thieving. If you tell your thief "I can't buy this today" they will find another fence and you might lose that thief forever. Maybe more thieves if they find out they are a better fence. So you just keep buying stuff, even if your not selling it nearly as fast.