I have Graco Sweetpeace Infant Soothing Swing.

I had 2 actually. I sold one on CL a few months ago for $90.
That one was pretty much new. I got 2 as gifts and instead of returning one, I just gave it to my mom so she could have something for LO at her house.
Well, LO hated it and so we sold it and bought LO some toys for grandma's house instead.

He hates ours too obviously, but we've used it more than my mom's was used. How much should I sell for?

All parts are there. It can be plugged in but it also battery powered. It has an iPod/iPhone connecting dock. It can double as a bouncer with a vibrating switch on the seat. It's seat comes off and a Graco car seat can be clicked in to swing in its place. It retails for $168.00 right now (was $180.00)
One issue: it has a small crack (can't tell when looking at it unless you inspect it) and it does not affect the function of the swing at all. The last photo is a pic of the crack.