there are a couple of main items to consider when evaluating a scissor lift.
1- the condition of the scissor arm assemblies, wear in the arm pivot pins and wear pads.
2- the pothole protection system operation, wear in the linkage and weather it deploys properly when the platform is elevated.
3- the control box on the platform, the joystick condition, weather all functions operate properly.
4- steering linkage and operation, linkage should not be worn and steering should operate smoothly.
5- battery tray condition, most trays will have some corrosion but make sure it is not excessive, replacing a tray assembly can be expensive.
Insure it has the correct type and size batteries installed, they ARE considered part of the counterweight system so skimping on installing the correct batteries will play a large factor on the stability of the unit.
6- tires, they all should have tread on them to some degree, tires that are worn smooth will not grip well on ramps and could cause loss of control of unit if it breaks traction (specifically on a wet surface).
7- platform guard rails, none are bent and all safety pins are in place (especially if it has an extension platform installed). the safety pins are usually attached to a lanyard that is fixed to the guard rail frame using a (keychain type) ring. if there are none then they will have to be installed before the unit is able to pass an inspection. (one of many safety items checked during an annual inspection).
If you have ANY doubts find an aerial lift technician and have them check it out before you purchase it.