Maybe just a few simple checks:
Both lasers working? Correctly aligned to the actual cutline?
Are the 90 & 45 degree mitres accurate to the fence (simple cuts required on inexpensive 300mm sheet material) checked against an accurate mitre square.
Is the turntable base correctly aligned with the other contact surface: fence, wings, extensions.
Is the material clamp present?
Is the Mitre-halving device/guage still present? Slides out of lower LH end of the substructure.
Is the slide & plunge actions light & smooth, or rough & grumbly?
Does the blade guard retract correctly when plunged & then slide back (down) smoothly & safely as the head assembly is raised?
Is there a correct 260mm blade fitted? How are the teeth? All there (60)? Sharp? Sufficient "meat" left in the wolfram carbide tips? Not exactly a deal-breaker if not, except that the genuine OEM blade is horrendously expensive (pick a fair price & quadruple it!) & therefore may be grounds for an additional discount if deficient.
Is the blade arbour rigid or rattly?
Is the "smooth (slow) start" of the motor nicely progressive, or is there a jolt on startup? How's the speed control? When you adjust the speed down whilst running, does the speed slowly decrease, then "lock" on the lower selected speed?
Does the blade tilt (bevel) adjuste operate smoothly? In both directions? To both LH & RH bevel stops?
Does the Mitre turntable turn smoothly all the way to both stops?
Is the blade change & adjustment allen key onboard? Located on rear.
Lastly, & probably most obviously should've probably been first, is the blade assembly, when correctly aligned to zero/90 degrees bevel actually perpendicular to the turntable/bed of the saw. Not a huge drama if any of the alignments are (slightly) out, as it's normal for any saw to drop out of critical alignment over time, and most parameters are eminently adjustable.
Lastly (again), is the owner's manual present? More than just about any other Festo/ol, this is especially important to describe, illustrate & demo necessary adjustment & correct alignment techniques & procedures. It's probably available online if not, but it's good to have the original to ensure part numbers are appropriate to your particular tool's production date.
By no means a definitive list, but these are what I'd watch for if I was buying again, & based on my own rather limited ownership of the saw about 15 or so years ago.
Others perhaps more familiar with the saw may suggest other things to watch out for.
