Buying a used dry suit is difficult. There are so many measurements that need to be correct for it to fit properly. Not only does the suit have to fit your body but the wrist and neck seals need to fit. Most suits have boots, those would also need to fit. A $300 suit that needs new seals and new boots could easily cost another $400. If you feel comfortable doing your own repair the components are about $250 (seals, boots, glue, tape.)
That said I dive with a well experienced diver who bought used and has done very well. The wrist seals didn't fit so has to use dry gloves.
Most suits will have latex seals. Stretch the seals and look for checking (lots of small cracks) also look for gummy or sticky areas. Either one means the seals will need replacement soon. The zipper is the most expensive repair, $400, so take a close look at it. Don the suit and check to see that moving your wrist and neck don't create channels in the seal. They should be snug but not too snug to stop blood flow.
You can test the suit easily. Some people use plastic bottles to plug up the wrists and something larger to plug up the neck. Or you could don the suit and inflate from a scuba tank. Inflate the suit (I inflate with a leaf blower inserted into the zipper) and spray the worrisome areas with soapy water. Make sure you bend the zipper, like it would be bent when donned and check for leaks.
Also keep in mind that you'll want undergarments, a dry suit hood (or cut the bib off a wet suit hood), dry suit gloves (or cut the gauntlet off wet suit gloves), and the boots of a dry suit may not fit your fins.