If you play, hassling with moving an old Steinway to the restorer and back may be worth it.
But if you don't play, you can't even assess the device yourself for condition without paying an expert. Bad business plan. There are as many experts as phone numbers, and most of them are trying to sell a nice new imported piano because they make so much money by employing serfs to do the work. Legions of salesmen will be telling your potential customers how stupid buying an old piano is.
Since I quit working and started following the stock market more closely, I've more than doubled my retirement funds in five years. That is a much better investment. A better use for your time, and no hernias required moving stuff around.
I've got some old pianos, and enjoy listening to them play. If you want collectables, find something that you know more about. And be aware you have to fill out special tax forms for the 28% rate if you actually make a profit on one. You can't hide a restored Steinway as a cash transaction, it would be more than $10000 and require reporting by the banks.