Hi,
I second what Graeme has said. Talking about the optical tube only, the most serviceable type is probably the Newtonian reflector, then the Schmidt-Cassegrain, then the refractor type, some of which need a specialist to dismantle.
The Dobsonian mount is a very simple type to tune up, and there is a great deal of stuff out there on servicing equatorial mounts like the EQ5. When it comes to the electronics of Go-To type computerised mounts, it gets a bit more difficult, but the thing to do there is to get a demo at the time of purchase.
It is rare to find yourself with a lemon if you buy here on IIS, I think (although it has happened), and there is a lot of expertise also if you need advice. I am using my 5th scope now, all bought 2nd hand here (being cheap my ownself
) and they were all good buys.
Cheers