As with any trials machine the general condition of these bikes will say a lot about the bike. If it looks very shabby with broken mudguards, tatty paintwork, worn tyres etc, then unless the bike is very reasonable money, or wanted for spares its best to steer clear and look for something better.
Common faults to look for on both TLR and RTL are pitted corroded fork stanchions (most recent Jap imports suffer from this), corroded wheel rims, noisey motors, slipping kickstart ratchet mechanisms, worn/leaking rear shocks.. Obviously over and above these specific things you need to look at whether or not things like tyres and chain/sprockets need replacement.
Parts availabilty for the 200cc bikes is pretty fair, but some parts for the 250cc machines are difficult/ impossible, and this is something that does need to be considered if you are thinking of buying a bike needing work, or that you intend to restore/rebuild.
Finally I would say if you intend to bid on any trials machine, that unless you are able to carefully inspect the bike before you bid, that it would be a very good idea to very cautious about the description of the bike, and try to speak to the seller on the phone and arrange to pay when you collect the machine.