It's kind of tough with the big boys, the 34-footers. In many things, the bigger the better hence, the more valuable. But many people, especially those buying trailers for the first time, feel that their needs can be best met by a little one.
So, when they are new, they do cost more than other TTs, but the funny thing is that they lose value faster than others.
You are asking what is a good price, and is $21,000 it. I would say that even though it's only thirteen years old, unless it is in very good shape it may not be. You are already saying that it needs tires , but you need to look deeper than that; tires could be bought for about a thousand dollars for that TT. The peeling clearcoat is normal for its age and although it's a pain to have, it does not impede your use of the trailer in any way.
Take the time to do a detailed look
by downloading and printing out (all the pages of) the Trailer Inspection Checklist
If you can do it with one of the local inspectors that you see listed on the right side of the portal page, so much the better. But even if you can't, after you do the list, look at it and take your guesses as to what it would cost to put everything on that list "right".
Then, why not go back to the buyer and share your findings with him (or her)? If you can sit and look over the list with him, you may find him reducing his price without any haggling.
Be patient, work through it, talk to the owner/seller.
And good luck!