If you are doing the Amazon Marketplace thing, here are some words of advice from a third party who has just seen some Marketplace drama go on.
First of all, advertise only books that you actually can ship. There's no point advertising something that you do not have.
Next, if someone then orders the book you do not have, do not send a different book on the same subject. Contact the customer by e-mail (you know, the near-instant communication medium that we use on the Internet) and explain apologetically to them that you do not have the book, offer them the similar books that you have in the e-mail or offer them an immediate full refund.
When you have not followed the above advice and sent the book unsolicited (and slowly) with a cover letter explaining that you have sent it as a replacement, be prepared to get a negative reaction - both on your Marketplace rating and in the feedback. You are engaging at that point in a form of inertia selling which is both unprofessional and a legal edge case. You have broken a contract that you entered into. Legally, you owe the customer money and you forfeit any right of ownership to the substitute book you have sent. Read the above linked law - The Consumer Protection (Distance Selling) Regulations 2000 s.24.
Then, having pissed off the customer sufficiently, don't then play the victim card. The fact that you are a charity makes no difference - everyone who is selling things needs to be customer-focused, regardless of whether they are doing it to buy yachts or to help feed the third world. Trying to emotionally blackmail the buyer for leaving you negative feedback (and leaving an insulting response to the feedback on your profile) for your unprofessional conduct only goes to prove the point they were making. In this particular incident, the seller described how one of the people ended up crying as a result of reading the feedback provided. Not to be heartless, but this isn't primary school.
Similarly, leaving an angry 'seller response' on your Marketplace profile is neither big nor clever. If you are an Amazon UK user, avoid buying books from scawapache, since he has yet to master the basics of customer service.
If you have got this far on the route, treat it as a learning experience. Learn that treating your customers in an unprofessional way is not in your best interests, especially on the Web where they can talk back. Don't emotionally blackmail them - learn from it and don't do it again.
