TIRES AND WHEELS
First off, please realize that tires on bikes are disposable. The fresher the tread, the better they ride, but mountain bike tires are soft rubber—you ride them hard in very abrasive terrain, and they wear out. So expecting to replace tires occasionally is common. But knowing whether a bike might need $150 worth of rubber might be nice while negotiating a price.
First, look at the overall condition of the tires. How fresh is the rubber? Look for any signs of cracking along the tread, bead, and sidewalls. If there is cracking in the rubber, the tire is approaching the end of its life. Also note the width of the tread. Does it seem appropriate for what you are doing? Typical trail riding will use 2.0- to 2.3-inch-wide tires. The harder and dryer the trail conditions, the smoother the tread; the softer, looser, or wetter the riding conditions, the deeper the desired knob profile.
Now inspect the rim. Along the bead of the tire, do you see any dents or dings? These are typically repairable, but a flat spot is typically not.
Spin the wheel and make sure it looks straight and round. Put your hands on the spokes. They should all feel even, without any being ultra tight or super loose.
Are the wheels setup tubeless? If so, tap the valve-stem to release a little air and see if a little sealant quirts out with the air. If it’s totally dry inside the tire, you will for sure need to add sealant.
Part and Labor Estimates:
Wheel True (labor): $15-30 and up
Tires: $35-85 each
Tubeless Sealant: ~$15
Rims: $60-$100
Spokes: $2-5 each
Wheel build (labor): $50 and up
NEXT: HANDLEBARS, STEM, SADDLE, SEATPOST, ETC.