First of all, let me begin by telling you why you should listen to me.  I have been collecting movies on VHS and DVD for the last 20 years.  During that time I have amassed a private collection in excess of 2500 films.  I never was a dealer or seller and collect solely as a fan.  I've shopped around quite a bit online and at stores and I genuinely feel that this information will help you do the same with confidence.

1. FIND WHAT YOU WANT!:  When searching for a specific film, make sure it's the exact one you are looking for.  Many Ebay sellers are auctioning thousands of titles at the same time and can be vague when describing them.  If it's a Special Edition you're looking for, read the listing to see if it is specifically that edition.  Many movies have been remade and re-released in different forms, so check the running time of the one you want and compare it to the listings run times.  If a regular version runs 92 minutes and the Special Edition with extra footage runs 96 minutes make sure the one you're bidding on is 96 minutes long!  If you cannot find what you want in an Ebay store or listing than wait.  Check back every week for new listings and eventually you'll find what you're looking for!.
2. BUY THE REAL THING!:  Before bidding/buying the DVD you want, make sure it's legit.  Many seller's who offer movies at extremely low prices are often selling bootlegs or copies.  This is illegal and if you're not careful, you won't know until you received your movie that it's a fake.  Try to avoid buying DVD's from out of the country.  The main reason being that they may not play on your unit due to region codes (U.S. is Region 1).  Copyright laws are federal laws and do not apply to other countries.  Sellers from the Phillipines, China and Korea are infamous for selling these knock-offs.  If a foreign seller is your only option than check their feedback.  Make sure it's high, check to see how long they have been selling and try and contact a past buyer about the product.  Make sure it's a studio release, factory sealed (unless USED) and in it's original case as well.
3. SHIPPING:  The average Ebay DVD seller will charge between $4 and $6 shipping for a single movie.  Check the seller's listing for combined shipping discounts if you plan on buying more than one title at a time (which I recommend).  If a seller is charging more than $6 for shipping than you're probably not getting such a great deal.  Say you find the movie you want on Ebay for a Buy It Now price of $4.99.  Compared to the retail store price of $14.99 that's a nice discount, but wait - shipping is $8.95!  There goes your deal, but you already clicked the Buy It Now button!  Many seller's make their prices more attractive by making it up in their shipping charges.  Also, when buying more than one title make sure to ask the seller if they will ship together.  Some seller's have a few warehouse locations and your movies may ship from different places at different times.  This also may result in the seller not offering combined shipping discounts.  Looking into this ahead of time avoids any confusion after you've made your purchase.  It is up to you to be aware of such things.
4. WATCHING YOUR MOVIE:  Try and view your purchase within a week of receiving it.  Most sellers offer a return policy for defects or damages between 10 and 30 days of purchase.  If there is a problem, contact the seller directly to try and resolve it.  You'll be surprised how helpful and friendly most sellers are in such a situation.  Many matters are usually resolved within a week or two and everyone is happy.  Only if the seller is unresponsive or rude should you involve Ebay.  If the movie arrives in a timely fashion and plays without a problem, please leave positive feedback.

I hope the above information helps make shopping for DVD's on Ebay a little easier as well as satisfying for you.  I love to hear from other collector's and fans, so don't hesitate to contact me.  You can check out my movie review page in the aol homepages directory under my email address rockyhorror74.  Thank you!