Always ask whether the DVD is a Master? which basically means is the picture quality good - Is the sound quality good? This should help to avoid any misunderstanding although this very much depends on the honesty of the vendor and also the vendors understanding of quality.

Quality would normally be described in percentage terms. So for example if you wanted to buy the latest blockbuster movie which is not even out in the cinemas yet then the quality will normally be less than 100%. Some shops may even let you see the quality if you ask nicely. VCD's are now roughly 50 Baht or so but can be had a cheaper if you shop around.

DVD's are also readily available and cost anywhere from 100 Baht. You should note that original DVD's can cost as little as 240 Baht and there are quite a few shops around Bangkok where you can buy original movies in DVD, video and VCD format. Most vendors will give you a box for free if you ask, while some may charge 5 Baht per box. If you have a DVD player at your hotel do check each DVD for quality and if you are unhappy then you should have little problem changing the DVD.