I got an old 350 for $10 at a garage sale .....
It depends on what comes with it. If it is just the filter body, I wouldn't offer more than $10 ... It will cost you at least $50 to DIY all the rest of the plumbing to make it usable.
If it is in really good condition (check all the gaskets! those go for $20 if you have to replace all of them), with all the hoses & the quick connect sets (x2), intake & outflow, media basket & filter (+/- wet/dry & gravel vac attachment), then maybe $40 is a fair price. <Considering that it is $85 new at Big Al's.>
I personally find the Magnum 350 very finicky to use & prime, & it is not as versatile as the Filstars or the Fluvals, so I wouldn't pay too much for a used one.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.