I'd definitely send pictures of the damage and stamps. tell him you feel misled and see what he says.

In response to your actual thread title, which I never actually commented on earlier

If he tries to make amends and sounds genuinely apologetic for not mentioning the stamps and other damage, then in the end I'd probably give a Neutral.

Negatives tend to sting on eBay, especially if it's not an actual business and doesn't have high turnover. He has already done a lot for you by investigating prices at the post office etc. so he has put some effort into being a good seller.