These old machines are not worth anything in terms of trying to sell them at garage sales. They can often be fixed up, but that is usually worthwhile only if you can do the work yourself. All it may need is a new belt, cleaning and oiling. If the brushes in the motor are worn, then you need someone who understands motors to take it apart and fix or replace the brushes. It is generally not worthwhile having a repairman do this work as it will cost quite a bit more than the machine is worth.
I routinely buy old machines at thrift shops and fix them up for donation to a local charity. My husband would be able to install a new belt, and he has even taken motors apart to adjust or repair the brushes inside (which can get worn), but I try not to buy machines that need this amount of work anymore. I wait and look for newer machines that appear to be in working order. The most I pay for those is $30 at the thrift shop. I no longer buy machines as heavy as the one you have. The newer ones are lighter and easier for me to transport.
A machine such as yours can be a workhorse if you are able to fix it up. Since the machine didn't cost you anything, you can feel free to take it apart and learn about it. Not everyone wants to spend the time, though, and it does help to have access to someone who is mechanically handy. My dh has helped me many times when a machine had me stumped.
Oh, and you can still purchase additional feet that will fit these old machines.