You have to look for these thing:
1. You should know or try to find out how old is your piano, what’s it called, find it’s serial number, what type of piano it is, what’s it called.
2. Does your piano need repair, is it tuned, has its pitch dropped, has it sustained any water or sun damage.
You need to know these things before you can start selling your piano or if you are buying one you should request this info.
History: Is the piano tuned and serviced regularly and you have a record of it? If you have kept the receipts of when the tech has made a visit and looked at your piano, then fantastic. It’s valuable info for the buyer, because he can contact the tech and confirm it’s not fake.
Knowing the original cost of purchase can be helpful in setting the sale price and knowing your pianos worth.
If the piano was restored, then the receipts can show exactly who did what, for how much and can determine the future sale price and is possible to check the quality of the work done, without them it’s hard to prove anything or what was even restored.
The state of the piano will improve the price of any piano, yet the worth has substantially more to do with inside of the piano. It can look good, but if it’s rotten inside, then it’s pretty bad.
If you have no paper trail, then you should ask professionals help and get an evaluation, it’s the best course of action in your situation as you can’t check the service receipts or anything else that will tell you about the history of the piano. Your local piano expert will surely know more than you.
Your piano might go up in price if it has any unique/rare/interesting markings or engravings on it.