Well I bought a slightly wrecked 99 regal and put her back together and with some paint the car is now ready to sell and looks like nothing happened. But, the car randomly decides not to start. I've tried jumping the battery and I swapped the starter with my brand new one from my other car. Neither fixed it. When I turn the key all I get is a click from what I assume is a relay behind the glove box and nothing else. Then randomly it'll start but I can't sell it like it is. Is there a common issue? I assume starter because car is older and has sat but that's not it... Is there a safety switch that's messed up?

Thanks for any help, have a man that wants to buy it but can't sell as is

Dale