As Seely said Merckx frames are very nice and draw good money. If your seller sells cheap you might think about it just for resale if you don't like it. I don't think I've ever seen a "legit" (as opposed to Falcon made) Merckx sell for less than $500 and most well over. If the one you have a line on is from 1986 it should be a Merckx made Merckx as I'm pretty sure Eddy was making his own by then. Check decals to be sure.

Also agree with Seely about avoiding a tri-specific bike. Don't know if Eddy made one but if it's got different sized wheels, down-sloping top-tube, etc it won't give the ride you're looking for unless riding in a straight line on aero bars is what you want. Again if it's cheap re-sale is a consideration, there's a strong market for Merckxs.

Consider you may not want to keep riding sew-ups so there's wheel considerations but the ride difference between a Merckx and a hi-tensile steel framed Moto Mirage is night and day, and I like old Motos.

This link gives some general info: http://sheldonbrown.com/vrbn-g-n.html#merckx

FWIW I've seen some Falcon Merckx's I thought pretty decent but am near positive an '86 vintage bike would be the Belgian "real deal."