A used harness can be a sensible buy if it still fits you and the safety-critical parts are in good shape. Harnesses have no electronics, but they do wear — the webbing, stitching, buckles, spreader bar, hook or rope and the closure all take load every session.
Check everything that carries the kite's pull before you commit. The webbing should be free of fraying, the stitching intact, the buckles and closure smooth, and the spreader bar, hook or rope should not be heavily worn. A cracked shell, frayed webbing or a worn rope-slider setup is a reason to walk away or budget for replacement parts. Whatever the price, the harness still has to fit your back correctly.