A used softener can be a sound purchase, but its condition depends on things you cannot see from the outside. Before you pay, separate what you can check on site from what you have to take on the seller's word, and get the unknowns in writing.

Ask about the water it was running on

Ask what the water supply was and whether the unit treated well water, city water, or water with iron. Iron, manganese, and some sediment can coat or foul the resin inside the tank, and that damage is not always obvious. A seller who knows the water history can tell you what the resin has been through. A seller who does not know should lower your expectations.

Check the tank and the brine tank

Look for cracks, bulging, rust, or stains around seams, fittings, and the base. In the brine tank, look for a hard salt bridge, sludge, or evidence of standing water that suggests a leak or a bad float. Clean and dry surfaces are a good sign, but they do not prove the inside is sound.

Ask for the control model

Get the control valve's model number and confirm that replacement parts are still available for it. A control valve that no one stocks parts for can turn a cheap purchase into an expensive repair. Also ask whether the control is timer-based or demand-based, since that affects how the unit uses salt and water.

Ask about resin condition

Resin cannot be judged well by eye. Ask when it was last cleaned or replaced, and whether the previous owner saw any loss of softening, such as soap that does not lather or spots that will not rinse. Ask for a water test on site if you can, using a hardness test you trust, and compare the treated reading with the untreated reading.

Confirm what transfers

Find out whether the seller still holds a warranty or whether any remaining coverage transfers to you. Confirm whether the unit was removed by a licensed professional or by the owner. Check with your local authority or your installer about permits and code requirements for the connection, because those rules apply to you as the new owner.

Decide before you pay

If the tank is cracked, the control has no parts supply, or the seller cannot say what water the unit served, treat the price as a repair estimate, not a bargain. Compare the cost of a rebuild with the cost of a new unit that carries a clear warranty. Get every promise in writing before any money changes hands.