Buying a windsurfing sail on Ebay isn't that hard.  There is certain information you must have to buy the right sail- size, recommended mast length (or luff), and boom length.  It helps greatly to know the year model of the sail.  Basically, newer sails after year 2000 perform much better than older sails.  After that, differences are very slight.  Buying slightly older model sails can sometimes be an advantage, as they were generally designed for longer masts.  For example, a 5.0 sail from 2000 could be designed for a 430 mast, whereas almost all current 5.0 sails need a 400 mast.  Buying the older sail avoids needing a 400 mast.  If you are at a level of sailing (low) that you are learning something from this guide, then you should not buy racing sails or sails with cams, they are a bit of hassle with relatively low gain in performance.  I don't think there are really any bad sails being made past the year 2000.  Probably avoid those sails strictly labelled as "trainer" or "kid" sails unless that is exactly what you want.

The type of wear and tear that might be difficult to assess is sun exposure, which wreaks havoc on monofilm.  Have an index of suspicion for sails that come from sunny locations, ie Hawaii.  Ask about the condition of the monofilm. Faded or brittle monofilm will tear catastrophically, you want supple and flexible.  Otherwise, what you see is what you get, for the most part.  Sails with imbedded fibers in layered monofilm ("x-ply") are considered more tear- and sun- resistant.

Windsurfers practice a pretty esoteric sport, so if you buy gear from an actual windsurfer, you are likely to have a positive buying experience from someone who feels a bond with others wanting to learn the sport.  Except for the large quantity of really old sails (and other gear), I think the quality of stuff available is generally very good, at a huge discount from retail, that will last you a long time.  Get the right mast for the sail.