Obviously this isn't a worldwide market, but for used phones or mac laptops it works.
$500 for a Mesa Boogie Subway Rocket amplifier--maybe, if it's pristine
"It has new tubes"--that are never standard Mesa tubes
"Just got it serviced"--never has a receipt
On an amplifier that the reverb is clearly not working, knobs are cracked or replaced with generics, the jack is flaky, looks like it has been thrown down several stairwells (at least try to fix the Tolex as that's cheap unless you try to do a full re-covering), etc.
And then they get really incensed when you show them that the last successful ebay auctions were all under $350.
Have some realistic expectations, dude.