When I was in the used book business in the Eighties, copies of this novel from 1981 were everywhere. Now I'd hate to have to try to find one, although they can be had on-line. It was probably so popular because of that sexy cover, and the salacious subject matter probably didn't hurt sales, either. BEST OFFER is basically a sex comedy about a group of suburban couples who decide to auction off the wives for a night to raise money and save the private school all their kids attend.
I haven't read this novel since then, so I don't know how it holds up, but I remember it being surprisingly good, with some darker, more serious aspects underneath all the wink-wink, nudge-nudge sleaziness of it. If you ever run across a copy, you might give it a try, although I make no guarantees. As far as I know, Robert Calder published only one other book, a horror novel called THE DOGS that I never read.