James Beth hit the nail on the head there; the problem is that it's not a bidding site, but "spec work", or as I prefer to call it, art slavery. Work for free in the hope that you may get paid–and don't think you can get away with a sketch to win the contest holder's heart. I got banned for submitting a sketch. I can however, get my membership back if I do contests to win back my membership.

Yep.

I do some work on bidding sites, primarily Upwork. It's not great, but at least you're not expected to work for nothing. The only unpaid work I do is writing sales pitches and consultations.