For a starter uni a Torker dx is great. Assuming it’s in decent shape you are highly unlikely to break it. Those unis are known to be quite tough. The trade off is that they are heavy which is why they are considered inferior. To me that sounds like a fair price on one if it’s in good shape.
You’ll probably find that a 20" wheel is small for muni. It won’t roll over rocks and roots as well as a larger wheel and would also be very slow on easy flat sections between technical features. But it would work for practicing skills you’d use in muni. I agree with Shmolagin that it’s a good price if it’s in really good shape.
The Black DX is a solid, heavy uni. I have seen one frame cracked at the seat tube, but it was by a rider much further along than a beginner after a couple of years of abuse.
As far as the warranty goes, even with new trials unis, there isn’t a whole lot once they start getting ridden. Check the cranks and pedals to make sure they’re tight. Spin the wheel to make sure the bearings aren’t destroyed and the rim spins true.
The picture in the Amazon link is a non-ISIS version. It’s not a problem unless you destroy a crank. The cranks that come on that uni are 125mm. Many trials riders prefer 137s or so. It’s difficult to find anything other than a 125mm or 150mm crank (way too long) to fit that spindle. The wheel in that pic is also a 48 spoke- strong, but heavy.
We keep a couple of older non-ISIS Torker DX unis (a Trials and a 24) around for loaners. They are solid. They are heavy too.
As far as the price goes, if it’s a newer one than the pic on amazon with the 36 spoke wheel and ISIS cranks, I’d be more inclined to spend closer to the $150 on it. If it’s the older model I’d stick closer to the $100-$125 mark.
Torker DX was my first unicycle! It worked really well on trails, and it held up to my temper tantrums (the learning process was frustrating for me, and it wasn’t uncommon to see me chuck my unicycle across a field or kick it to the ground). However, now that I ride a lightweight 26" muni and I’m venturing into trials, my Torker is WAY TOO HEAVY. Yes, its strength is comparable to a tank, but keep in mind you will have to hop up things with that tank. Maybe it’s the weight weenie in me, but I’d go with something more lightweight if you plan to use it predominantly for trials. If you prefer more muni, it’s awesome for a beginner. I will never get rid of my Torker, just for nostalgia’s sake.
I have that unicycle. I’d say get it - for the price, it’s a good starter unicycle. You may want to get a different seat eventually though - I just got a used fusion freeride seat on ebay for about $35 shipped. The seat that comes with it can make you pretty sore.