Yeah, merchandising your books might feel like you're selling out, but in an odd way you're actually honoring your fans by giving them more than just the book to keep, or the story to remember. You're giving them a physical, tangible piece of your world to hold onto. I mean, if movie posters will sell for anywhere from $20 upwards to several thousand, then you know there is a market for them out there. A little 50c piece of paper that's hung on the wall and sold for a couple of bucks is an easy money maker and fans will love you for it!
Oddly, it might even increase your sales by having the merchandising items act as free advertising for you. Imagine how many new fans are made simply by someone putting a poster of something up on a wall that someone else sees and likes.