I am confused as to whether the online business of horticulture should focus on selling the product in the initial days and after getting the user traffic expand its services to other sellers by adding a "sell with us" icon on the website.
Bear in mind that the approach Amazon took was during a time in which e-commerce was still just taking hold and was far from commonplace.
Of course, starting with your own products and building an audience of shoppers gives you some leverage in attracting sellers when you change to a marketplace environment. The downside is that in the short term your product mix won't be as robust, and you'll rely solely on your own efforts to promote your site.
In the early stages a few enticements come to mind:
1. Early mover - for those early sellers, all important reviews and ratings will stack up and give them a lead over later adoptees of the platform.
2. Additional exposure - if you don't impose exclusivity on your sellers, they stand to benefit from having additional promotion to customers they may not otherwise reach - ie while the upside may be questionable - there is no real downside to listing products on the site, assuming that the time burden of doing so is low for the sellers.
In addition to what Ryan mentioned, you have to remember that Amazon started from a very narrow niche. They did not start as the global e-commerce destination they are today, but rather from enabling people to buy books online. And only books. This gave book publishers and even bookstores with an additional channel through which to sell their products and consumers a new way to buy books. So there was both a great new, revenue-stream for sellers (and still is today) as well as a new, convenient way to buy for buyers (and still is today).
I am a strong believe of focused marketing. Amazon sold books in the early days as you know, and they were amongst the first of not the first to sell books online. They identify themselves as the platform to sell BOOKS. Once they have a massive double market place and became successful at selling books, they then reach out to other sellers. I remember in the early days I was not interexted in buying books on Amazon, but I did know about them. Once they added DVDs to their platform, I was hooked. Start centric with a focus demographic or an area of the market you can dominate, and then expand out to the next set of products by perhaps surveying your existing database of customers. "Hi Mr Customers, we noticed you have been buying books from us for awhile now, if we start selling DVDs on our platform, would you buy it?". Don't be afraid to ask your customers for quick survey. Perhaps offer them a $20 coupon towards their first purchase of the new line of product you plan to sell after plants.