I do auctions and BO was more of a pain because I get offer that are like $5 more than starting price for an item that will sell for a few hundred more. When I used fixed price, rarely, I do use BO because that cna get a conversation going that can lead to sale.
As for low ball, there is no set-in stone definition. In part because it depends how much the seller is asking. I have been known to offer 50% less or even less than that and secured a buy because the sellers' prices were so far from market, and I have been able to make my case. When someone out $1500 on item that's worth $350 they should expect so low offers. But I buy and sell in antiques and collectibles. In other categories, I think offers need to be more in line with what things are really selling for. If a phone you are interested in selling for $800 and the seller has a price of $1000, make the $800 offer because if they don't take the next seller you go to likely will.
