Given what you've said above, I think this is flat out scummy... but then again I have now seen the photo and that's professional quality. No wonder why he wants to use it- it sells plants for him. I wonder what will happen when somebody buys their plants and doesn't get flowers that look like yours in the photo?
Why don't you call eBay on the phone. I've done it before over a seller trying to play games and I had phenomenal results- the whole listing was pulled in all of about 1 hour. They most assuredly do favor their sellers but they are a business and don't want to end up in messy litigation- litigation costs $$$. Also too, if you have an e-mail address to eBay, send them a second request and consider using this service when you do-
http://www.readnotify.com/
Matter of fact, if you've got this seller's e-mail address, why don't you consider CC'ing him utilizing ReadNotify too?
Claiming his employee used the photo is lame, he's the employer and is ultimately responsible for most acts of an employee.
In many states, ReadNotify's certification of delivery is acceptable to the courts. Sending them an e-mail correspondence that is certified and adding comments to the effect that "this is the second time you have made eBay aware that the photo in their seller's listings are being used without your knowledge and consent and that you would like to see your photos removed from the listing to avoid this situation escalating to yet another level" might get eBay's attention.
While you're at it, you probably should seriously consider sending this guy a written request to stop using your photos via common carrier. Include that this is not the first time he has used your photos to sell his plants and that you want it to stop. If he has a fax machine, fax your correspondence to him too and definitely send it to him certified mail also where only he can sign for it. If he refuses to sign for it, no biggie. Just hang onto the return receipt that will have the word "refused" written on it. At the bottom of your correspondence to him CC like this-
CC: common carrier (regular USPS mail)
fax (include the fax number)
certified mail
your attorney
eBay
Next time he uses one of your photos, you can contact eBay via e-mail (using ReadNotify) and tell them this is repeated and habitual by providing them with copies of your correspondences and maybe that will expedite the process of getting your photos removed from his listings.
Use certified delivery for mail and e-mail. You will be creating a paper trail.
What's really sad about this whole situation is that you strike me as being totally sincere. A plant in exchange for the use of your photo is totally reasonable because I don't think there is any way that guy could get a photo of that quality without contracting someone to take the photo and that would cost at least a couple hundred dollars. Professional photographers aren't cheap for a reason. I feel really bad for you.