A transmission rebuild (or any car repair for that matter) should never fail after only 20 miles of driving! Based on the list of parts they recommended you have fixed (that you said no to), the spark plugs are the only one that would cause an engine to stop working. There is definitely a chance that they made a mistake. The problem most likely has something to do with the rebuilt transmission in which case, it is the mechanic’s responsibility to get your car running again.
The first thing I would do is go back and discuss the situation with them. Ask them to repair your car and get it back in working order. If the shop is honest and has a good reputation, then they should be willing and happy to take another look to see what the problem is and fix it.
Did the shop offer some sort of warranty or guarantee on their work? Most reputable shops do. A typical agreement will state how many miles they guarantee the rebuilt transmission will work properly. If the shop refuses to help you and only offering to buy your car for pennies, they could be trying to scam you by taking your money for the repairs, making sure your car failed shortly after and then offering to buy it for far below it’s value. In this case, I would contact the authorities such as the Federal Trade Commission (FTC) to report the scam.
If the first shop won’t help you, I would get a couple of other repair shops in your area to take a look at your car, tell you what’s wrong with it and give you a quote. You might be able to get the engine working again for the same or less than you paid the first time.
If worse comes to worst, then you can definitely get more for your car on a classifieds site like Craigslist or by contacting a company that buys junk/scrap cars.
