I contacted someone at a book review blog a while ago. She was backed up regarding reviews on behalf of the blog, but she offered a review swap in which we'd purchase and review each other's books.

She eventually told me that she couldn't write a positive review of my book ("Becoming a Hero: The Complete Collection") and didn't want to hurt my chance of sales by writing an honest review. I thanked her and posted my review of her book (which was mixed).