Need advice - Felt F1 frame

That looks like an early 2000ish frame based on the paint and interface from the top tube/down tube to head tube.  $350 is a good deal for a frame, but I question what might be wrong with it.  Overall, it looks like it is in good shape.  You can't compare it to today's F1 frame (which is several thousand dollars).  So long as there is no known or visible structural issues, you can't go wrong for that price on carbon.  Just be careful with a visual inspection.  My guess is the guy upgraded his frame and is just trying to shed the old one.

Your Marin will likely need a few new components.  Your crank might not be compatible with that BB and the front derailuer that you have is probably clamp on.  You'll need a Braze On for that frame.

looks like a nice frame. but the main 2 questions are (a) is it the right size for you ? and (b) does it have any structural problems in the frame or fork ?

if both questions are cleared, then might be a good buy. You might try to get it a little cheaper.

It's not too hard to build up a bike like this, but be careful on the cost. Components add up fast.

I did the same thing, bought a complete Ultegra group off EBay (around $850) and popped on the components (also a Felt frame similar). No problems and ride it ALL the time.