Hi Shrimpy, see no one has replied as yet, so I will give you a few heads up.
In the main forum Board index, scroll down to the bottom for the Bikes for Sale and Dealer bikes advertised on this site.
Alternately looking at internet searches for Trials in online sites like Ebay, Trading Post, Gumtree, Bike Sales, Digital Bikes, and try Yakaz (ebay search listing), reveals a lot of bikes not listed here, but experience in buying is needed to know if its worth buying from unknown persons outside the trials scene and clubs.
Links from dealer add websites from this site will find on their pages a variety of brands and prices for secondhand bikes, backup, service and parts.
Not knowing your budget or state locality, getting to a few club events can provide a good insight to whats around locally.
Its hard to answer your question not knowing your skill. I would recommend for a first trials bike something less than 10- 6 years old for an entry bike for price alone. For capacity, dont go above a 250, a 200 is better and 125 is a great leveler to learn how to use the available power. Remember the bikes are developed for torque, not power, so capacity is for experienced hands, not beginner. Go for a 2 stroke, being easier to maintain and budget to repair, alternately the Scorpa 160 or 200 I think is a great 4 stroke for learning on also. When you skills get better, by all means, get what you like and go up capacity if you like. This first bike does not really have to be a current model, skills learnt on an older model still apply, the first bike will get trashed more often too, so a few scratches and bent or broken parts wont hurt so much.
Hope this helps!

EDIT: Seeing there are some nice older and good priced bikes older than 10 years old, some bargains to be had. Check details like last rebuild, type of grade owner rode, maintenance etc. It could be a good thing or a lemon. My first bike was an ex A graders, and cost me a lot more later to fix all the abuse. Try and get Clubmen of C Grade ridden bikes if you can.