I would stick with private individual sales. That one I posted for example. The guy will probably be able to give you a good account of his time with the car, past owner(s), issues-if any and receipts...etc.. I am sure you know all that.
I have been peeking around at a 10K car to mess around with since my wish for a new 997 S is on the waaaay back burner for now.
Found a 944 Turbo with some professionally installed engine mods out in southern Cal. PCA member since...blah blah blah. Clean as hell 70K miles. $10,700. I like the styling of both the M3 and the 944 but this P car appeals to me more than another M3.