Of course rust issues can change everything
I had a friend who was looking for a bike and he stumbled across a titanium Serotta in his size on the local CL for $675. My advice was, "Go now with cash - inspect all the welds first but if the frame is clean, don't bat an eye- pay up"
He talked to me again several days later stating -- "it's hard dealing with you cyclists"
When I asked how the Serotta deal went down, he said , "Well I tried calling the guy, even left a message that I'd give him $375 -- but he never called back"
---- when I asked why he would do something boneheaded like that, he replied -- "I just don't feel like doing a deal unless I'm screwing someone over a bit "
Ultimately the guy got sick of looking and paid FMV for a nice Ironman that fit
-- just a cautionary tale against trying to get more blood from a stone on an already very good deal-- But again, if rust is bubbling around the bottom bracket or something - tough to say