Festivals are a great place to sell DVDs of your film, but will the Festival let you? It's probably a good idea to inquire in advance. Will you be able to set up a table outside the theater? Will you need to have a website in order to sell them? Will you need to have some one do the fulfillment? Figure this out before you show up.
Your film screens and everyone loves it. They want their friends and family to see it too, but there are no more screenings left. Your audience loves your film, how are you going to mobilize them into action?
People that buy your DVD at a festival are your core base and they want to help you out. Give them your card and ask them to email you. Get theirs and email them. Let them know that this is a special sort of DVD they bought; tell them that is a DVD for house party use. Let them know that if they can get a certain number of friends to come over (25? 50?), you will do an iChat with them live for an hour and discuss how you made the film. Let them know that you will get them more of these "House Party DVDs" for their House Party that they can sell on your behalf and keep a cut for themselves. Trust people; it will do more for you than the harm the few times you do get ripped off will hurt.