eBay and Craigslist finds

Whoa, thank you for the information. Expectations are mixed. I just have a sudden itch for another bike/ more refined steed which I totally can't afford that I can't seem to explain.

Learn something everyday, i'll keep my peepers peeled.

I don't think you'd be the first person to see 'Merckx' and think you've found a bargain, there is a market for frames like this.

If you're after something cheap then do what I do and set up saved searches for 'include description' (531, 531c, 653, 753, 631, 853, 953) and also (slx, sl, tsx, columbus) etc etc. Sometimes you just have to be lucky though (put stuff in brackets like that, separated by commas and it'll search for all of them).
I had a huge drought recently where I didn't find anything but for sub £80 I've had in the past - an 853 frame, a handbuilt SLX frame, multiple 531c, 653 and 753 frames, a couple of Bob Jacksons, and other ridiculously badly listed awesome frames.

Just keep an eye out, sometimes things slip under the radar and you spot it!