4. Sail Colour
It may be stupid but is does it look bleached out? This means it was exposed to the sun a lot, which reduces the lifetime of a sail massively. So you should ask yourself if the price is good enough for a short term sail. Obviously second hand is second hand so it will always look used, this is where your research helps. How much is the sail at full retail, how much are similar sails selling for. Be aware that one brand may sell better than another due to the materials, manufacturing process and demand.