If you work like that, then you'll just sell one domain instead of several at the same time. If people do as you say and bid for example 0.1BTC without mentioning the domain name, then who knows what domain that bid is for? Other people will simply refrain from bidding since it's confusing them. Why not simply let people bid on a seperate domain, and if another person is interested in that same domain, the bid will be increased. And of course it will allows people to bid on multiple domains at the same time. Target is to sell plenty of domains instead of just one, right?