While tightness and regularity of weave are important, the quality of the wool determines the resilience and lasting patina of a rug. The quality and fastness of the dyes are also important. However, the quality of rugs from India, Pakistan, China, Turkey, Tibet and Nepal has improved in the past 20 years to the extent that many Persian-design carpets from these countries are finer than the Iranian originals. In addition to the factors above, the demand and price for Oriental rugs is highly dependent on design and color trends. Today, even the finest of them are selling for a fraction of their original price.
Classic designs tend to retain their value reliably.
Simply stated, age and condition affect the value of used rugs. My best advice is to shop and compare rugs and rug dealers. Check out the range of prices for the types of rugs you love. Decide where you are comfortable doing business. Look for established firms that rely on repeat business for their success.