Its best just to buy new, then you have a 2 yr warrenty and don't have nothing to worry about. Buying used equipment etc you take the risk of everything, batterys going bad etc and some of the batterys are expensive about 30 bucks for the battery to replace int the transmitter, 15-20 for recievers...it adds up quick. Best buy new, then you have the warrenty and tri-tronics is real easy to deal with if you have warrenty claim issues.
Hey check out the Collar Clinic. If you do a search for Collar clinic you should find it. They have alot of good deals on refurbished collars and I believe they provide a warranty. I train retrievers and know a few people who have ordered collars from them and have had no problems. You could also check out ebay, they usually have some decent prices as well.
If you want old, obsolete , refurbished equipment then check with collarclinic....I don't know why anyone would want an old style shock system with the changeable points and external antennas...there not worth the money IMO. I have talked to the collar clinic and yes sometimes they do get the newer style in but by the time they refurbish it, they will sell a 2-dog system to you for 350 bucks....and coming straight from the sales ladys mouth...I repeat, your better off to buy them new. Those old style receivers with the changeable points and antennas you could'nt give me one. If you live in thick brush and hunt often enough ...you will brush whip those attenas so bad you will end up replacing them every season...and with the changeable points...I geuss thats alright if you only like one level of shock at all time....having it adjustable on the transmitter is worth the price in gold for me...you can lower it down for handling issues or turn it up for trashbreaking.... being able to change the level of stimulation witht he turn of a knob is worth the extra price of buying new.