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.