a 98 yz125 in great shape should fetch around $950 to $1200 with a high end around $1500. if it was rebuilt, ask for receipts. ask to pull the exhaust off and inspect the cylinder. ask what has been done to it. new plastics, seat, pegs. yz125 and 250 have been known to crack the frames in two. check for cracks and fresh welds. check out the air filter. if it is dirty, chances are the bike has been neglected. if it has hardware special nuts and bolts, that's a concern. with it on a stand, hold the seat and lift the rear wheel up and down. if it moves at all, it needs suspension bearings. check the rotors for severe ware. bad rotors need replacing along with pads. just my 2 cents. dirtbikes are a big gamble.