My rule of thumb is this: If you advertise that your art is for sale, you should put it up for auction, or put a sales price on it. If it is not advertised for sale, then it is incumbent upon the potential buyer to inquire if it is. At that point, it is appropriate for the owner to say: "I'm not looking to sell, but make me an offer." At that point, you make an offer, or explain why you've changed your mind.
If you advertise that your OA is for sale, and you get inquiries on the price, it is not OK to say "make me an offer." I dislike that tactic. You want to sell it. You should be up front about how much it would take to part with the piece. If you want multiple bidders on it, then put it up for auction, don't play games. I also dislike the tactic of saying it is for sale, but not publishing the price and forcing people to make a specific inquiry about it. An unadvertised sale price just opens the process up to manipulation. You may give one buyer one price, and another a different one, knowing they are a "whale" or that they need the piece to finish a book, etc.