08/07/2022
My suggestion would be to use Shopify. There are a few others, WooCommerice and BigCommerce are relevant examples. However I tried them and found Shopify to be much better for the appearance of the site. There is good standard themes which are easy to change plus you can use basic code to update.
1. Shopify has a 14 day free trial and then various prices from there. Its around $30 per month plus a small percentage of sales for the basic (but sufficient one). Increases from there.
2. You handle the delivery and return. Shopify has a good interface to manage this with the orders but that's your responsibility. Unless you dropship (having a supplier that will send out the goods for you - useful but there is a cost associated).
3. That's for you. Social. Paid. Content. There's lots of methods. A lot of articles on this out on Google. Shopify again have a University section to help teach you.
4. You need to source Vendors. Alibaba is popular.
5. Depends on the circumstances. They'll want a minimum order number.
1 & 2 are relatively easy. 3, getting people interested in your product is difficult. 4 & 5 could be problematic. Depends on the product.