Spring= $25-30(I can't remember what Sean and Kyle charge for used springs) market it to guys looking to do the double d mod
Pressure plate = $20 - only really useful to someone who wants to mess with trying to change the color or completely rebuild something something.
Give away the retainer ring to whoever buys one of the other parts or just destroyed theirs in a "hold my beer and watch this" incident.
Chrome brake arm = $40-50? I got my billet one for $50 and the only thing wrong with it was some tape residue.
Another good way to price stuff is to search for Pinwall cycles on ebay and knock 15-20% off whatever they are selling stuff for. They are very proud of their used parts, but are close to or higher priced than just seeing if Sean, Kyle, Patrick, or maybe Wayne (One2dmax, Captainkyle, patmax, NHVmaxpower) have something sitting around.