I might be wrong here, but I don't think so.

You bid 300k for an item, you get outbidded. The gold will get sent to your stash, except that it won't be 300k, it'll be 255k. So the 15% auction fee does not just go for selling, it also goes for bidding.

The other option is that when you bid 300k, you will not actually bid 300k, though I don't know why it would be like that.

If there is a bidding fee, I don't see doing anything but a buyout making any sense, at least if you have a bidding war with someone else. I didn't put this in the General area of forum, since I think that this must be a bug.