It's hard to put a price on older hardware. It also depends on what the parts are and what "tier" they fell in when new, how old they are, and original pricing. Sadly most probably won't come near the price you paid for them new, especially if considered "obsolete."
( geofelt said this first, I'm seconding his/her post.)
eBay's completed auctions and "buy it now" posts can give you a good idea of market value in general. (Sometimes a bid frenzy may push higher than normal, but that should be fairly easy to spot.) Search REPUTABLE sites for for sale/trade forums too. (PCPP has such a forum area.)
As far as selling, you got eBay, CL, and even for-sale/trade forums on various REPUTABLE sites and even on FB. (i.e. the closed group: Computer Systems/Parts For Sale) Some like the mentioned FB group have a requirement beyond just being a member of the group, such as having a heatware ID.