Restringing an autoharp will bring very considerable improvement in sound quality and should be done much more often than most people are willing. The improvement in sound is the same as a guitar with new strings only magnified by six.
BillD is wrong wrong wrong ( but no disrespect meant) about string changing
Also, much more information would be needed to accurately valuate your autoharp since people unfamiliar with the instrument often do not see flaws when trying to sell online. The rough value is about 100-200 dollars if it is a 21 bar 70's model with everything intact.