Discoloring on used surfboards is normal. The discoloration is usually caused by UV and it is mostly cosmetic and will not in any way affect its performance.
You should also check the fins of the used board you plan on purchasing. Check if there are any cracks especially around the base if they are glassed on. You have to remember that cracks on the fins are expensive repairs. If the fins are removable, you should check the boxes for cracks, water seepage and stripped shrub screws. You have to remember that fin boxes are also very expensive to replace and getting one with one of the mentioned damages means spending a lot of money for repairs.
A little wear and tear is natural and is really not a big deal if timely repairs were done as it will prevent extensive damages. You also need to remember that used surfboards looses its flex characteristics. However, this feature will not really mean any difference and if there is, it will only be a minor one.
The best deal that you can get for a used surfboard is getting one that has hardly been used. The best used surfboards that you will be able to get are from people who didn’t see the board they purchased as a perfect fit for them. They will want to get rid of the board fast in order for them to purchase a better board that is perfect for them. Who knows? Maybe the board that they didn’t like would be perfect for you.