Fish aren't kept in those 'jam jars' for very long at all. And although I disagree with the use of them in shops, I would rather see a betta in a 'Jam Jar' that gets it's water changes daily, than in a tank full of fish that are ripping it's fins to pieces.
Or in the shops around me they have 'betta walls', it is a wall with an external filter system.
Each betta is in a clear perspex box about 6" x 5" x 2", with an inlet for new filtered water and an overflow outlet pipe that goes back to the filter.
As it is only temporary and the shop does have to make money, I think these homes are ok.
But what you are selling isn't a 'temporary' container. You are advertising it as a suitable long term home.
A terrible buy on your part, and I think you'll find yourself stuck with them (No one here will buy one anyways!)
You should be trying to sell them as 3l biospheres or something. And suggesting people fill them with and aquatic substrate(Which you could supply in a measured amount), and then planting them with miniature aquatic plants(Which you could also supply, or at least supply a list of. Now THAT I would probably enjoy having on my desk and would consider buying.
