I deal with a lot of graphic designers now and over several years. I own a consulting firm that provides business, marketing, advertising and sales consulting to small-to medium-sized businesses throughout Canada, USA and the Caribbean.
What I usually do for a creative start-up or re-imaging project is this:
I contract 3 graphic designers to give me 10 creative designs (including logos, slogans, etc...) for a guaranteed 3 hours of payment at the going rate - anywhere between $60.00 to $120.00 per hour depending on the designer. This way I get 30 creative designs for a set price - these designs belong to me and I can sell the one chosen by the client. However, what happens to the other 29 pieces of design that belongs to me?
These ar all redundant anyways. Because I cannot use these in anything unless I get another client with the same name or initials, etc... that I can use another piece - the chances of that ever happening are so impossible. What I do with these redundant designs is the archive. I suppose that if I had purchased 10 designs from you I would take offense to you trying to sell one that belongs to me - intellectual property and all. If it belongs to you, however, you can do whatever. Please be sure that it belongs to you before you sell it.