An okay condition 3120xp brings in around $750 around here, a really clean low hour saw will fetch $900... it all depends on the condition.
BTW, my 3120xp is smoother than the 880. I haven't raced them yet, but the power feels to be about equal.
3120 much better buy used. 088/880 hold value better. I've seen 3120s on CL as low as $500 but may be hot.
My opinions based on my stock 880 and a couple 3120s.
Power very close. (I've run my side by side many times in same wood. I use them with 42 and up bars in big wood.).
3120 much smoother to run.
Depending what year, the 9.7k 3120 limiter is annoying to run. (my 880 is 12k+ limited)
Fixed jets are unsatisfying. If you buy an older 3120 then I would make sure it has at least a 1.12 mm jet.
880 is a little louder and harsher (but a nicer sound)
dogs to chainbrake clearance for big wood is noticeably better on 880.
880 feels a little lighter and more compact to use somehow.
3120 uses common bar mount,big stihls a specific stihl large mount
3120 has snappier throttle response
long chains a little easier to swap on 880 at first
Bottom line is that if you can take the vibration, then the 880 is maybe more satisfying to use (better built and designed machine I feel), but I generally grab a 3120 as it takes less toll on me. Like them both, though.