I'm looking back at years and years of buying "vintage" on line -- both tube and SS.. Sometimes I've received very defective equipment listed as "Works Fine". Sometimes I've received very stupidly packed equipment, broken (think McIntosh glass and loose vacuum tubes for starters)-- and wondered whether any thought went into the packing process. The feeling of receiving a dented box that rattles and clinks is painful and angering -- not a happy way to have a hobby.
By the time I get the stuff repaired (transporting, waiting, PAYING), I've spent a good deal of money and time, and endured no small amount of agony. AND, "virgin" vintage equipment will about always benefit repair/restoration -- caps and tubes -- to name two candidates. More money and more time.
I've pretty much decided to buy vintage in post-repair condition from local shops or well-reviewed eStores (see below). But more than that, I come to think that new gear will be more convenient, more capable, and cost less when completely accounted. New tends to be reasonably auditionable.
If a person really wants to buy vintage on line, I'd agree that TMR is a reasonable starting point, as is Audio Classics, along with a number of the NY shops/salons and one or two in California.