'M.H.',
Depends on what he wants for it, and what you plan to drive it with.
Nothing wrong with the AL-811H's if driven correctly and used for SSB or CW. They are not exactly made for AM, will work there, but don't expect much at all, nothing like their usually stated output for SSB/CW. All of the '811' amplifiers are made more for the lower HF bands rather than for 10/11 meters. Very nice on 20 meters, slightly less than adequate on 15 meters though, sort of.
And then it just depends on what he's asking. If it's less than half of the new price, snap it up and re-sell it if nothing else!
The AL-811H uses something between 30 and 100 watts for drive. Both 'ends' of that are sort of 'pushing it'. Especially the 100 watts thingy on AM, I just wouldn't go there at all.
Something else I definitely would not do is 'over-volt' the things. They have a variable input transformer tap, meaning they can be powered from around 100 vac to about 130 vac. One trick is to adjust taps for using 100 vac when the actual input voltage is closer to 120 vac, makes for higher plate voltages. Also makes for more heat build up. Have a friend who's modified AL-811H sort of melted the tank coil's supports. More of a 'U' shape than straight. Luckily he caught things before experiencing the "OH SHIRT!" results. Bad enough on 120 vac, you can also do the same thing (but much easier) on 240 vac. Also one reason why 'lifting the hood' on used amplifiers is a very good thing to do before 'plunking' any money down, and not just on the '811's.
- 'Doc
[It's not the 'H' model but I do have an '811 amplifier. Wouldn't really get rid of it (unless offered some ridiculous amount of money), but don't think I'd get another.]