Either you didn't have a 'fresh proxy' and it was flagged or you had a delayed ban.
Either way, you don't get banned simply from logging into OSBot - as far as I know, anyway.
I would think that a ban is a pretty harsh punishment for something like this. What happens when the person goes inactive for a while due to unexpected irl problems? What happens if his mule or main with all his money on it gets banned? What happens if the person bid on the wrong account by accident? What happens when the person was expecting something different foot some reason and bid on the account then later realised the account didn't have certain requirements he would like?
Should he be banned in those situations?
Same with the seller. What happens if the account gets banned prior to the end of the bidding? What happens if the seller doesn't like the winning bidder and either doesn't sell to him or mald up an excuse like the account is banned in order not to sell? What happens when the two traders can't agree on a MM because one of then is trying to get our of the trade?