I think selling locally might be the best way to start. See if local shops will sell some for you. ,your local garden centre may be happy to sell your locally grown plamts....talk to the manager. Can you sell from your nan's place? Maybe keep most of your stock at your allotment. Sell popular plants. , Growing polyanthus for example. You can display a whole range of them in the brightest colours that attract anyone passing by. I think it better for you to decelop your selling skills at home...get used to talking to people about plants, get an idea of what you can charge, see what,people want and will actually buy. Take cuttings of things like penstemons....we all love penstemons.....geraniums, pelargoniums, etc.
You can develop an online business when you have understood a little more about supplying a market. You wont have postage or packaging costs either.
I used to sell plants myself on a small part time basis....it was very profitable and I was close to purchasing a run down nursery locally. For me, I have a reasonably attractive garden and it did attract people. They could see plants I had for sale actually growing well in my garden and they would point to them amd ask,if I had any for sale.
If you get a good knowledge of plants and their needs....how to grow them, etc.....you,will get a reputation for being able to advise people about plants and not just selling them.
On line, Ryan, I think is best considered after you have had a year or two of practical local, on the ground experience. What do you think?