Are you referring to mechs in general or old-style tube mechs? I'm merely referring to old style tube mechs without constant contact fire buttons. In general, those don't seem to have anywhere near the market as all other mechs. But I gave up on VU and never go near FB so maybe there's a larger market for non-constant contact older tube mechs than I thought?
What's been mentioned above; there's a market but it's just not on this forum which is pretty quiet. VU and FB have the most activity, being allergic to Facebook I have an account signed in there to do nothing but participate in a few FB groups for vaping gear.
Mechs are basically metal tubes, they're worth whatever someone is willing to pay for them. Super T and GG mechs are what allowed me to transition to vaping full-time back in 2010, I have a pretty large collection of Super T and GG gear, Caravela, Ba Gua titanium (beautiful presentation, horrible mod), GP Paps, Nemesis, etc, etc, etc, many of which cost ridiculous prices at the time due to scarcity. None of them are available anymore, so the price amounts to whatever someone is willing to pay. If it's a casual user, it's doubtful they're going to pay too much because who really wants one? I've used all of mine, then just cleaned and packed them years ago and switched to Dicodes.
An important consideration for collectors would be the state of your mod. Many people who actually use their mechs have pretty beat up gear, because... heavy metal meet gravity! There's a considerable difference between pristine quality and something dented with scratches all over it.
The only people likely to pay top dollar for branded old-school mechs are collectors; nobody else is really going to care or understand why a battery holder is worth $500.