Sorry to hear you lost it...but you have to act fast. The seller took the money...can't blame him. He didn't necessarily know you would buy it for sure.
That is typical the case. For popular good quality lens, if you buy it used (in good condition), you should be able to sell it back for about the same price you buy it from. I bought my Tamron for $300, if I sell it for $300 after using it for a year, I bet people still want to buy it. So next time, if you see a good popular lens like that for sell, if the price is right, you should get it. If you do not like it, just sell it back.