Sounds like your friend got the deal of the decade. Using that as a measuring stick, you are going to be disappointed. For any item, there are very knowledgeable people like David above watching continuously and ready and able to pounce when they see a good deal. So while people like me are still studying the ad, trying to figure out if it is a deal, a knowledgeable buyer is picking it up.
I do the same on bicycles. I have been buying, restoring, and selling vintage road bikes for ten years. I have enough knowledge that I can spot a deal from sketchy information, and have the time/tools/aptitude to take care of any deficiencies. I am active on a vintage bike forum, I no longer post what I pay for bikes (deals) as it leaves the impression that its easy to find a $300 bike for $25. Its not. And like David, I look continuously, no stone unturned. And I have made some lousy deals too. Fortunately, bicycles don't take up the space of fiberglass trailers, and don't take the cash either.
In the fiberglass trailer market, I have no such knowledge or skill. So I know I will have to pay more for a trailer. I would rather get a solid trailer from a knowledgeable and fair seller, than a project from a random person that "inherited uncle Charlie's RV".
+10 on trailer weights. Manufacturers weights all seem to be low, and of course, don't include any of your stuff and don't include options the trailer might have. We have a Honda Element, very limited tow capacity. For that car, I figure my motorcycle pop up camper (350 pounds) is about right. No way I am going to pull a fiberglass trailer behind it. Meanwhile, my F150 can easily pull something in the 5000 to 6000 pound range.