I'm hoping that someone with knowledge of Marshalls in particular can provide some details. Here's some pointers that apply to any used tube amp.

[u]Ask some questions up front.[/u] How old is it? What kind of life has it had? What kind of music was it used for? Has anything been replaced or modified? How long since its been running? What warranty is on it?

[u]Look it over.[/u] If it looks like it fell down a flight of stairs, it probably has. If the tolex gleams with freshly-applied Armor-all, you gotta wonder what it looked like before it was cleaned up.

[u]Warm it up and look some more[/u]. Be sure the Stanby switch is OFF; then turn on the power switch. Take a look at the back of the amp while it warms up. You should be able to see that the filaments are glowing in each tube. See where the speaker impedance selector switch is set. Combo, or head & stack; the transformer setting [u]has[/u] to match the load.

[u]Listen[/u]. There should not be any sound from the speakers yet. Now plug in your guitar and turn on the Standby switch. There may be a pop or thump sound. After that, you should hear nothing! Mute your strings and turn the amp up. There may be a faint hiss, and a small amount of hum, but not much. Moving your muting hand a little bit should let you know how loud the amp wants to be. Turn it down (you get to make lotsa noise later :) ).

Play a bit with the amp set clean. If you have to change any of the settings to get this to happen, note the control positions before you move any of them. It should sound good. If you know what your guitar sounds like, you should recognise what you hear, without any strange colouration. Adjust the tone controls, to see what effect they have, and to hear if they make scratchy noises when they move.

Happy with the clean sound? OK, return the controls to where they were and see what happens. Marshalls don't get rave reviews based on their clean sound. The owner probably had it set for his idea of the 'sweet spot' where the distortion was just right. Even if this amp is in a store, the salestypes can't resist honking on the latest sample to hit the floor. So let it wail!

If you like what you hear, and nothing smokes or smells funny, you've probably got a keeper. Its time to dicker on price.