Hey, also a plant lover here. I also have experienced great frustration sourcing plants and pondered a similar platform. Haven't acted on it as too busy with existing projects. Issues not dealt with in your platform: plants with no known source should be hidden by default, shipping issues such as international and interstate restrictions on mailing plant material should be recognised, time and cost should be respected including currency exchange rates, and the options of growing from seed or spore should be in-scope.
If you want to grow it, my suggestion would be to use a social model, so allow people to store information about their own garden and then share a feed of events like "I planted this!" or "This flowered!" or "This reproduced!" or "I collected seeds!" to draw people to your platform. Sharing material and the use of endemic plants should be encouraged where possible, not just outright commercialism.
The gold standard today for accessible all-species info at the global level is iNaturalist (pulls in nice maps plus taxonomy plus Wikipedia, though unfortunately does not really delineate in the map between nominal natural range and current range). Not sure how you do it now but frankly it would be somewhat superfluous to attempt to reproduce such information independently, for example by maintaining your own image database.
Other things that can draw people to a site would include seasonally appropriate gardening tips (plant or fertilise or prune X now before Y season, check Z for A/B/C pest situation, clone P now, etc.), a relevant local events feed (which opens up potential travel revenue streams such as hotels, flights, group tours, etc.), and academic and publishing news in related areas.
Many of the nurseries have problems maintaining up to date stock lists. To commercialise, it may be useful to help them do so. To motivate them to get on the platform, you could for example allow parties to sign up for future purchases of crops not yet matured but with an estimate readiness date, thereby assisting the grower with cashflow and sales pipelining.