The best way to get buyers to buy more than one item is to have a wider range available so they're more likely to find more than one thing they want.
I notice some of your plants are listed under health & beauty. Anyone searching for those plants is not going to search that category for them but will look under plants, so they won't find them. You could possibly be reported and get a violation for listing in the wrong category too.
You've got in one listing "combine postage and safe". The first thing I'd do is correct your spelling and put save instead of safe, but I'd also give them an idea of the savings they'd make by buying more than one. For example, if you can fit three plants in a parcel under 500g, say so. Sometimes you may be able to fit four in but not always, so you could perhaps say "three will post for the same postage price as one, sometimes more, but please contact me for more details". I'd also say something about waiting for an invoice if they want to save on combined postage - they may not bother reading it but at least you've covered yourself.
I think you said once before your plants are different sizes and weights, but for ebay you might have to pick the plants that are similar sizes so that it's easier for you to work out postage and so you can make a definite statement about postage costs for multiple items. Once you get a bit of practice it'll be easier.
