If you do not want to keep the car I think you are going to lose money doing any work to it... have you not gotten your money's worth out of it yet ?
If it were manual trans it would be different.. but still not enough to go into the engine and do a major fix before trying to sell it.
Anyway, my guess is that it has to do with either the piston pin or rod bottom to crank situation... which will not be fixed by valve work.... If I am right then you will have that extra work into it WITHOUT it being in good shape... I suggest you part it out , or save it till you get more parts cars or something... but not put good money into an AUTO trans 240.... about the lowest demand of any MB I know of.... a Manual trans being on the other end of the spectrum.