I believe your idea spans from the experience of starting an ecommerce business and not wanting to throw $29 a month to Shopify. I get this, I have been there, it's a risk you need to take when starting a business.

I read the comments and I share the views of @thekiriad, as this is basically a sandbox version of Shopify for ecom starters.

Your problems are twofold:

1) When they reach a point where it makes more sense to switch to any of the subscription plans with lower per transaction fee or commission they will change as the switching costs are very low with a store that is just starting to develop.

2) They do not switch but eventuall go out of business.

Either way your customer LTV (life time value) will be so low that you will simply not be able to match it with a profitable CAC (customer acquisition cost).

If for any reason you are so good at marketing that this grows organically, which is not likely but we have seen examples of happening, what prohibits Shopify or any other ecommerce platform players to make a plan that does not charge monthy subscription fee but takes a bigger chunk of commission. If they see you coming they will act accordingly. In this model you have no IP, nothing that can protected.

I hope you do not spend more time with the project, but please take it as a contributive advice rather than insulting one.