In the past i have sold alot of second hand cars and swapped on occasions,unless both parties have verbally or written something on paper,there is no obligation to swap back,only done as a guesture of goodwill.I got stung on a car i bought from the buy and sell a year ago.Looked fine to me,traded it in to garage for my current scooby,3-4 days later got a phone call from the garage to say that the car i traded in was in a serious accident a few months back and did i know anything about it,obviously not, as damage could only been seen on ramps.3-4 days previously,the salesman had checked out the car,test drove it and said.....very tight car mate....... Paperwork had already been sent etc and nothing the garage could really do unless i agreed to swap back.
2 years ago i had bought a 01 galant and found out from a bit of investigating that it had not got 55k,but actually had 127k on it before the garage had bought it fron Merlin auctions.On this occasson i got my money back and some.
Bottom line is ,if you bought/sold privately,there is no legislation to say that you are responsible.unless you knowingly knew something was wrong and sold it promising it was 100%,this also is impossible for the other party to prove
Hope this helps a little
Alan