A guzheng is not an investment. It is a wooden structure plus a great deal of hand labour, and the second-hand market prices condition, not age. Once both sides accept that, the transaction gets a lot simpler.
There is no standard discount. Four things set the price: whether the soundboard is cracked or separating, whether the body is dugout (which holds value better than a laminated body at the same original price), how hard and how long it was played, and whether the parts are complete — original bridges, a stand, a cover. Two instruments that both cost 10,000 new can differ by half second-hand if one spent three years in a living room and the other five years in a teaching studio. So "what percentage do used ones go for" has no answer. Each instrument has to be looked at.
Pricing, if you are selling. Look up what that model costs new today and discount from there, rather than starting from what you paid. After a price rise, the buyer compares against today’s new price, not your receipt. Good condition, complete parts and paperwork let you hold the price up. A touched-up finish, a restring or a short soundboard crack should be stated plainly — honest listings sell, and the alternative is a return and an argument that costs more than the discount would have.
Four extra checks, if you are buying. First the soundboard: sight along it at a low angle in raking light, where hairline cracks and filled repairs show up, then tap it with a knuckle — a dead, hollow thud suggests a separation. Second the nuts: check the front nut for movement and cracks, because replacing one is not cheap. Third the tuning pins: turn each one; a pin that feels gritty or slips back will not hold pitch. Fourth the bridges: worn feet and cracked bodies mean a full replacement set, which is a real cost.
Three questions to ask. Any repairs — a replaced soundboard, a filled crack. Any water damage: smell for damp and look for staining on the baseboard. And whether it has been refinished to hide cracks, which is the instrument that photographs best and troubles you most. A receipt and a serial number raise confidence by a level.
How to transact safely. Be wary of listings with photos but no video. Ask for two clips: every string plucked in turn, and a slow pass along the soundboard in raking light. For a remote sale use an escrow or cash-on-delivery arrangement rather than paying in full up front. Locally, collect in person and inspect before paying. For transport, slacken the strings, remove the bridges and use a hard case or crate — do not hand a bare instrument to a courier.
When not to buy used. A soundboard that has been repaired after a crack, an instrument that has been wet and mouldy, one with non-original bridges that will not hold tune, and any expensive instrument with no paperwork and no clear history. The repair bill on those usually exceeds the saving, and what you end up with is an instrument that plays but always has something slightly wrong with it.
The value of buying used is a better grade of soundboard for the same money. The risk is an invisible repair history. Work through the four checks and most of that risk is settled before you pay.