Are used amps a good buy?
If the amp is of reasonable quality to start with, then it's a good idea to buy used and let someone else take the initial hit on depreciation. If you shop intelligently and pay market price (or less) for your used amp, then you can often sell it for what you paid if you don't like it.
If you buy from a shop, they should do more than put a price tag on the amp and put it in the showroom. They should have a tech go over the amp to replace weak tubes and charred screen resistors - the two most common causes of poor sound. In an amp that's more than about 20 years old they should also do a cap job, replacing all electrolytic caps and any signal caps that are malfunctioning. If the amp has a 2-prong power cord, they should replace it with a 3-prong cord. If a shop doesn't do this before they mark the price, ask them how much their tech will charge to bring the amp up to spec as I've outlined. If they haven't done the work, don't have a tech, and you don't have a tech lined up - walk away.
You need to pay attention when buying a used amp, because you don't have a warranty. I wrote a short article on things you should look for. Another good indicator is to find out what kind of track record the same model has had with other players - the Harmony Central review database can be helpful for this.
Quality amps can last decades before requiring any kind of service beyond changing tubes.