If you are intending to buy a dvd from a seller in a different country then you need to be aware of the different dvd region encodings. In the UK we use Region 2 Pal format DVDs. I only sell Region 2 DVDs in my Ebay shop so anyone in the UK will be able to play any dvd that I sell. But what if you are in the USA, Canada or any other country? Well, I put a warning in each of my dvd listings "If not in the UK / Europe please check that you will be able to play Region 2 Pal format DVDs before bidding." The warning is there because not all countries use the same region codings for their dvds. For example, if you are in the USA or Canada, the dvds released in your countries will be Region 1 Ntsc format because that is the dvd standard where you live and, therefore, that is the DVD format that a normal USA and Canadian DVD player is made to play.
If you have a multi-region capable DVD player then you do not need to worry about the different DVD formats and you can enjoy any dvd from any country. Even if you don't have a multi-region capable DVD player it very often can be made multi-region capable by entering a code through its remote control (generally these codes are known as "player hacks"). There are places that sell information on making your player multi-region capable but I find that using an internet search engine such as google and searching the term "player hacks" together with your DVD player's model number usually brings up the required information for free :-)