Hydraulic splitters undergo great stresses and pressure in their operation so they must be built from durable materials and parts. If not they would quickly self destruct. Because of this and because of how simple these machines are they really don't wear out very fast if they are well built. Hydraulic components use oil as a transfer of energy so there is very little metal on metal and hydraulic components are naturally long lasting.
Since hydraulic log splitters can last for so long, purchasing a used splitter can be a good idea. Even if parts do wear out most of them are relatively inexpensive. I have three log splitters that have been in commercial service for over 20 years and they are all still using the original hydraulic components.
When choosing a splitter look for a frame built of thick steel, especially the main I beam. If possible test it first and make sure the beam doesn't flex much if at all when splitting a tough piece. Look for good solid welds and large high grade bolts.
If you have a good frame to start with it should last a long time. If you have to replace some of the mechanical components you can figure that into the price when negotiating with the seller. Here are a few things to look for.
The engine is usually the most expensive part of a splitter and also the most likely component to wear out. Make sure it is easy to start and runs smooth. If not it may need work or need to be replaced. Some engines can be replaced for $300 to $400 but some brands are much more.
Check the hydraulic components for leaks. If the pump, cylinder, or control valve are leaking hydraulic fluid it could be an indication that these components will eventually need to be replaced or repaired. Although sometimes hydraulic components can leak for a long time and still work fine. But hydraulic fluid is not cheap so it may cost less in the long term to replace the parts. Most hydraulic components don't cost much to replace anyway.
Leaking hydraulic hoses and fittings may need to be replaced but in many cases just tightening them will stop the leak. Check the condition of the hoses. Sometimes hoses may look bad on the outside but it's just the outside layer and the hose can last a long time. If the high pressure hoses have deep cracks or gouges replace them. A hose bursting and shooting hot hydraulic fluid everywhere is not much fun.
Open the hydraulic reservoir and dip something in it to look at the fluid. The fluid should be relatively clean and not black and should not have water in it.
If it's a towable splitter look at the tires and make sure they still have good tread and are not damaged. If you can, jack up each tire and spin it to make sure the bearings turn smooth and quiet.
This should give you some idea of what to look for when looking for a used splitter. The main thing is to get a good solid design. After that the components can be replaced easily even if you are only mildly mechanically capable. Just factor the replacement or repair costs into the price.