A forklift must be checked well and given a test run prior to buying it. This applies especially to trucks being acquired online. It is really important to detect potential problems with trucks acquired from businesses online.
Starting the engine from cold is a great idea. Note whether there is excessive smoke and the ease with which the engine turns over. This is specifically crucial with IC engine trucks.
The test run must include an inspection of the truck's performance in challenging situations. A skilled driver can drive the truck in reverse and forward, up the ramp.
You can perform your own inspection of an IC engine truck. On a clear area of floor, run the truck around for a few minutes while a trained driver tests the hydraulic functions. After that, examine the ground underneath for indications of fluid leakage. Even seemingly insignificant leaking can result in serious issues which would be costly to repair later on.
The masts and chains must be checked by a skilled person. Check with and without a load that the mast works smoothly. Watch for sticking or 'binding' of the mast. A warning indication is play in the mast channels or between the mast and fork carriage. One more is creeping forward or creeping down of the mast when loaded. These can be signs of issues with valve, seals or ram.
If the truck is not new, the mast might have been changed at some time. Make certain that the mast and any fitted attachments follow the rating plate on a truck. Make sure that the sideshift is not sticking when loaded.
When inspecting the forks, look closely at the heel thickness for damage or wear. The manufacturer of the truck specifies minimum thickness. A loose fit can mean the carriage or hanger points are worn. Look for cracks on the arms at heels and mountings.