I'm betting the one that doesn't sell has something wrong with it. Some of the plants are verrry good at hiding or camoflaging bad parts. If you take one of the non-selling ones apart piece by piece (by pruning a little at a time), you'll likely find a dead bit that you've missed. Sometimes you can see a dead part while the plant is developing, but by the time it's full grown the dead part is hidden completely or almost impossible to see. Or sometimes it blends almost completely into the soil or other foliage.

If you really can't find anything wrong, and it still doesn't sell, just trash it--it's not worth tying up a space in your greenhouse forever.