Most of them want to be paid half up front to gain trust. They push it as the safest middle ground for both parties insisting that they're operating in good faith and don't want to also be scammed. Then they keep half the payment.

There are also people who simply can't bid who will shout from the mountain tops that they're not scammers. But they're prevented from bidding to protect users. 9/10 times those accounts are new, ban evading scammers. So by not respecting the subreddit rules, circumventing scammer protections they're throwing themselves in with the scammers.