I would offer them $100 cash max. You would be surprised how "flexible" folks can be on these.
I saw an 1870's Willcox and Gibbs full treadle setup locally listed for $550. (yikes) but throwing caution to the wind, I sent them a message explaining who I was, why I wanted it so bad, and that I could only afford $150-$200. They replied in three days saying that they had considered my offer and decided that they would rather sell it to a person who would treasure it for $200 than accept the $550 that a person had actually called and offered to pay because that person seemed like a "dealer or flipper".
Some folks sell these because they just want them gone and do not care, but I have found a LOT of folks really care about these machines and DO want them to go to a "good home"
Give it try. Be honest, tell them who you are and why you care.
They might not give a poop, but it is virtually pain free to try.