I sell workout DVDs and VHS on ebay occasionally. What bids you get is determined by a few factors: how popular the instructor is, the opening bid you set, how old the video is, the condition of the item, your shipping costs, your feedback history, how many of the same item that are listed concurrently with yours, just to name a few.
Of course Cathe DVDs and some of her VHS get top dollar. Other instructors that I've been able to sell at decent prices are Charlene Prickett, Mindy Mylrea, and Christi Taylor. Surprisingly I got a really good price on Cindy Crawford (like 20 bucks, whoda thunk!). The Firm, Karen Voight seem to be a dime a dozen so you may not get much for them.
If you aren't sure why your listing isn't selling, you might want to find other listings for the same thing and track them in your watch list. After seeing a few of the same items from start to finish you'll be able to determine what the market will bear, where to best start the bidding price and even your shipping/handling costs.
You could also consider placing them in lots. You might get a better response if you offer a group of DVDs VHS by the same instructor.
Also, I always make my auctions end on a weekend. People have more time to watch and wait on the weekends so you are more likely to get two or more people bidding against each other (as opposed to 2 in the afternoon on a Monday). People also don't like to bid until the last couple days so if you just listed your items you might not see any activity on them yet.
Good luck