This is one of those things that unless you can see it work, you can't really know what you're getting. Some things are easier to fix than others. The pistons in the pumps are usually made of sapphire so if they're broken, they're quite expensive. If they're not broken but scored, you'll never get a seal and your flows will likely vary all over the place.
Here's a link to a pump maintenance kit from Restek:
http://www.restek.com/catalog/view/40351You can see that if you need to rebuild just that part on a 4-pump system, you're into if for almost $3,600 (US).
HPLC's have many threaded fittings in the flow path. If someone cross-threaded a fitting or two here and there, you can have perpetual leak problems. Leaks like this generally lead to inconsistent flows and thus retention times. If your times drift, you'll never know what you're looking at without a mass spectrometer.
Most modern chromatographs are pretty useless without a computer to collect and process data. Getting a computer to talk to your system can be a huge problem. You could spend a lot of time and money trying to get that worked out. We had a situation where we moved a system (no names) from one location in our company to another. We paid the supplier of the equipment to come in and get it working and they could never get it going. We never really needed the equipment to keep our work process going so it sat there on the bench for a long time. We finally sold it to one of those 3rd party rehabilitators-of-used-equipment companies. I have some GC's in my lab that if the computer fails, I will probably have to antiquate the instrument because I can't find a PC that will accept the communications board (A/D converter/instrument control). Perfectly good instruments will probably become useless because I can't get a computer. Sad but true.
Do you have analytes that absorb light or do you need something else? If you get a variable wavelength uv-vis detector, make sure it works. Lamps wear out so they're usually not too expensive because they're a "consumable".
Even if you choose to buy a used system, I would get new columns. HPLC columns are not as forgiving as GC columns and you'll very likely have trouble trying to make someone else's columns work. They're also not cheap. Anywhere from $400-$1,000 for a "garden-variety" C18.
These scenarios are what Tom was talking about as used equipment being a "money pit". A used GC is a little better risk as it's a much simpler piece of equipment. If you choose to buy a used HPLC, I think you should at least ask for a money back guarantee if it doesn't work as advertised.
Good luck. Personally, I don't think I'd venture into this arena. Maybe with a GC but I'd be more inclined to go new on an HPLC.