Hey Flood,
I think your website looks really nice. There is quite a few things that you could do to your site which have already been mentioned that should help massively with marketing.
1) SEO is definitely a must and things like using 'title' and 'H1' tags should be simple things that you can do yourself on your site. I used cubecart ages ago and I know you can change the title of each page somewhere. Perhaps you need to go to their support forum and ask how to do it. Otherwise do a search on the Internet and there should be loads of stuf online. Just search 'how to change title tag in cubecart' or something similar.
2) Before you do anything, you should really do some keyword research, Google have a free tool so you can see which keywords to aim for that fit your site. For instance, 'plant seeds' gets some 12,000 visitors per month! The google keyword tool is here:
https://adwords.google.com/select/KeywordToolExternal
3) Increase customer confidence. Use a SSL certificate or get someone to install it for you, display the logo on the fornt page somewhere prominent. Shoppers want to feel secure if they are paying online. I would also suggest putting the paypal logo up top somewhere so people can see it.
4) As for marketing, also consider putting a blog on your site and post on it a few times per week. Talk about your seeds, plants, business etc. Its a great way to generate traffic and you do see results with it.
5) Use web2.0 stuff like facebook. Setup a facebook account specifically for your business and network with gardners online or start a group on facebook for those interested in gardening. Stuff like that.
So, yes go for it, there is no reason why you should not far exceed your ebay income with your website with a little effort and time.
One last suggestion, do some 'article marketing'. Its very easy, its free and it works. Jsut search online how to do article marketing and you should be able to pick up the basics.
Once you have done as much as you can for your site, then move on to the next project. If you can't get this site right, then it your next one may not be much more successful.