For the most part, I try not to spend over $20 - $30 shipped for cylinders and try to buy them in lots. I have bought "used" cylinders and can be hit/miss for being decent or not. So, I try to avoid those unless they do look very clean and at a heck of a deal.
For the most part, try and get "new". Look and buy all year round, not the month before Halloween. Buying air cylinders can be interesting, as I have ended up with metric ones with milli-meter threads, or other thread styles versus good ole NPT. I am not sure what the best advice I can give as "good judgement" comes from "experience", and "experience" can sometimes come from "bad judgement". Some of my bullet point advice.
- Go with Pivot Mount air cylinders, and hole through versus pinned. You can always Nose Mount them if you have the application. Not easy to make a "nose mount" cylinder into a "pivot mount". If you don't know what this means, then go do some reading.
- My favorite size air cylinder is 1-1/2" to 2" bore. I don't have many props that are overly lightweight and would need speed versus lifting force. The stroke length could vary, would say I most generally use 6" to 10" commonly.
- If you need more speed (or size constraints) as a big cylinder is going to move somewhat slower due to more volume to fill, then take a look at the Force to Pressure to Bore size chart and go with the size needed. My rule of thumb is to be at least twice the power needed to move the prop.
- I generally don't buy clevises or feet for my air cylinders. More $$, lol. So, I tend to make my own. For the most part, air cylinder rod threads are fine threaded, so can buy those needed nuts at Ace Hardware or bulk up from McMaster Carr.