I would recommend asking about moth protection to any buyer of handmade rugs made of wool. It is a major problem many used rugs have. Buying rugs on ebay or auction may save you some money rather than buying in a reputated shop, but there is always the risk to bring in moths to your home that are a danger to any woolen product.
Reputated shops usually have their stuff well mentained - this means old rugs that often come in as a trade in are professionally washed and moth protected. This is protecting their investment (rugs) because the damage by moth in a high quality inventory would become a expensive problem.
It is also ok to buy an used Kashan in good condition, washed and with new moth protection. When you go around shopping ask the sellers to show you good Kashan rugs, no Ardakan or Meshed quality.