Sellers have all the power when it comes to unpaid items as long as they use those powers. Always go through the unpaid item dispute and have your buyer block  in place which prevents a buyer with more than one unpaid item dispute recorded against their account from buying from you.

How would buyers ghaving negs stop them from buying? Unless you have thge time and inclination to check the feedback of every bidder you would not know they had negs and even if you did that it wouldn't stop them using a last minute snipe or using BIN.