Following is a brief list of useful resources if you want to buy or sell a TL125 or related items. Since for the audience for the web-based resources can be (and is) world-wide I'd recommend mentioning your general location, and also if you are willing to ship (overseas, inside the US etc.).
The only thing I would specifically look for in a used TL125 (besides the general used bike items) is nasty rattle from the top end, possibly indicating a bad cam bearings. They can be fixed, but it will add some complications to the job. Often the reason is plugged oil passage from cylinder to head, so check carefully before re-assembly. I don't recommend using any kind of sealants in this location, just the regular new gasket.
The TL125 Message Board (link on the TL125 homepage) will get your 4sale or want ad viewed by audience interested in the TL. Note that you must mention price for any 4sale ads. Ads are free.
Trials Action site has a very nice Trials Mart feature (located under Interactive - Bulletin Board - Trials Mart). Much bigger audience than TL message board. Ads cost $20 (honor system).
Honda TL125 Parts and Resources Page lists several companies that have parts for the TL. Some of these companies, like B&J Racing or Best MC Repair, may have used engines, engine parts or even whole bikes for sale from time to time.
Forward Air will ship your bike in the US. I have used them several times, generally shipping and insurance for a small and inexpensive bike (like the TL) runs a little over $100 when delivered to their freight depot. Bike must be packed in a crate (which you can normally get for free from a local motorcycle dealer).