I can understand it would be frustrating, and most people would assume videos = VHS tapes, but at the same time the word video became synonymous with movie and to this day many people will say 'I'm going to the video store' when they hire a movie, even though there's unlikely to be a tape in sight, so there would also be quite a few who don't translate the word video to VHS tape. 

As you seem to have plenty of room in the titles, I would suggest adding the word tape to them - VHS Video Tapes would be ideal. You could also even have a pic of a tape outside of its cover, so that people who don't read all of the listing / description will have an immediate visual. 

Keep in mind I'm not suggesting it's your fault people are confused, just that there are a few things you can do to decrease the chance people won't understand what the listing is for.