Everything is worth what someone is willing to pay for it . There isn't a price list. You can only go on what has been paid before as an indicator. Pretty sure I have seen a Dengar palitoy mailer listed on eBay.com for 225 dollars (Buy it now) , so on that basis 200 quid sounded about right to me at least.
I honestly don't know about whether your friend is correct about them normally fetching 600-800, I have never seen one before so I guess they are rare, but perhaps next time wait until you get some more valuations before doing deals via private message before you even listed them on the sales thread? You ignored the valuations given for your nien nunb mailers, listing them for double what any sane person would pay. So why didn't you do the same for dengar if you thought that valuation was undercooked? Next time maybe list one on eBay in an auction to test the water before pricing. See what it fetches , then price accordingly? Good luck with the rest of your sales.
