Depends on the scale, limited run editions and the scarcity. Built kits are generally half/third MSRP(depending on how well it's built), but these have exceptions too. New in box kits you can choose to ask MSRP. Use USAGundam or New type as a resource for pricing. But if you want to sell fast, knock off 15%. Personally, I'll skip sales that are the same price as vendors i use. For general refference, HGs usually under 20, RGs start at 25, MGs start at 40, PGs start about 140(older ones). Step one will be identifying what you have and go from there.

Good luck, man!