The introduction of the DVD format has made VHS tapes somewhat obsolete. Many people are now choosing to rid their movie collections of VHS, deciding only to collect DVDs. The audio and video quality improvements are so drastic between the two formats that going back and watching a VHS tape after growing used to the superior quality of DVDs can be rather disappointing.
This doesn't necessarily mean that the VHS format is dead. In fact, there are still many titles available on VHS that haven't seen a DVD release quite yet. If you consider yourself to be a movie buff, you're likely still holding onto a few of your favorite movies that haven't been released on the DVD format.
One reason that many movies aren't available on DVD is because of rights issues. Sometimes, movies sit in limbo for years without any chance of a new release either on VHS or DVD. In these situations, the only way to purchase a certain movie is by looking for a pre-owned copy.
One of the best places to look for used VHS movies is actually at online auction websites. Many people use these services to sell their old VHS tapes for a bit of money to put towards the upgraded DVD version. When shopping for these movies though, make certain that you're purchasing a tape that's in the proper format. If you're in North America, you should only shop for used VHS NTSC tapes; if you're in the United Kingdom, look for used VHS PAL tapes.