that is the point.... sell what you think may not suit your tank or outgrow your tank space...
IMO... no need to go for a bidding system.... (it may create unnecessary rise in price of corals and generate a false value of what a coral may really worth, thus misleading beginning hobbyists)
if you want that little attention to your corals, my suggestion is to put a REASONABLE cap to the bid, let the buyer pm you for a buyout.
some suggestions..may want to consider a little