New vs used is not so difficult. Just compare with other goods. New car vs same type 8 years old and 300000 miles on it. What is the difference? How is value difined? On new products you have a quite equal price on most producers. The final selling price depends on the amount of rugs you buy from the producer (getting a better price on larger amounts), your cost structure and calculation structure. Looking at the mentioned dealers and rug selling websites, you hardly find anything of quality, most is low(est) quality merchandise. How can you expect to find quality there? Most customers look at the product, like the color, and if the price is reduced from 10000 to 800, everything is fine. If you expect to get real quality rugs or original pieces, there are a lot of good sellers to find online or in reputated stores. But there you pay a lot more than on the mentioned sites - and get totally different quality.
Why?
Take a new rug of retail value with a fair calculation of 2000$. Have it used for 20 years. In this time the pile wears, cats and dogs and children had little "accidents" on it, often the fringes and sides are damaged, or the foundation is rotten. After the years, the difference between the cost of bringing the rug back into a "as new" condition is far higher than the price of a new rug, lets say 2500$ for repair. What is the value of the rug? -500$. Well, you can still walk on it, but the value is gone. What a great invention is ebay. Before ebay these wrecks where fore the junk, but now there are acutally poeple who buy this! Since most customers only buy from the optic, and have no idea about the defects a rug can have, a big market for these junk rugs has been created. I will use another way to explain it in a picture:
1. A new car is expensive since it has to cover the cost of the production and the trade.
2. A good relativly new car is still expensive but lower than new.
3. A 8 year old car with 300000 miles on it, with lots if defects and a new paintjob might look good at first, but it is junk.
4. A 50 year old car in perfect condition is a collectable with a price higher than the same model in new.
So the ebay market has developed into the junkyard for rugs - from the 4 categories above, most is quality listed as 3. or worse. Ebay specialized dealers buy all the junk rugs in Iran for little money to sell them on ebay, since it is still a profit to make. And rugman was very present on ebay a few years ago. I think the quality of his rugs is a little better than what is sold by other junkys, but still there is nothing to kick a rug collector out of his shoes. He has a good antique wash for his rugs, that makes them look a lot better than they are. If he has higher prices, it is ok since the quality is a little better than average.
You might find quite good rugs from private sellers or small sellers on ebay, but most large quantum sellers can only make profit with selling lowest quality because their customers expect lowest prices.