first, check the rules here about making such offers.
you could browse places like ebay & see what people are asking- but then you also need to watch & see what they actually get.
location matters - what may sell for under $100 in one area may bring in alot more than that somewhere else.
check local shops, guild, see what people have paid for theirs. i know a lady who picked one up in GREAT SHAPE---not *used hard* for $75---i know another lady who paid over $500 for hers- in alot less shape...value is as dependant on condition as it is on who wants it-what it is worth to them...some people will hold out a long time for a good deal- some will pay most anything for what they want.
you say it was *used hard* you may want to try to locate ads with machines close to the condition of yours-
you could browse through the vintage machine topic here on the board & see what owners here have paid- what kind of deals they have found- that would give you an idea of what you could ask- though not really mean that's what you would get- when you meet the (criteria) to list here on the board someone may show interest- but ebay & craigs list are probably where you need to be checking---and locally- meeting & talking to people who own them will help you- and if you can show it to them they can let you know the pro-s & cons & condition issues of your machine.