Your best bet on figuring out value is going to be to watch Ebay to see what said parts are going for.
Another barometer is Craigslist and Facebook classifieds. I don't have a Facebook account, so no idea how that works, but for Craigslist to be an effective tool, you'll need to search well beyond your local area using a Craigslist search site like searchtempest.com .
If all you can find is used parts, I'd list anything new in box and/or never installed for at least triple, you can always negotiate down. If you have V8 motor mounts and frame mounts that are new, they're worth their weight in gold.