You buy a multiport modem, hook it up to a linux box configured as router, get a web host to host your site that advertises, promotes; and keeps track of users, passwords, payments, etc. There are many webhosts that will set up an ecommerce site as part of the monthly fee for hosting your site. You use one phone number for all users, but you need to purchase additional phone connections for each user (i.e. Each user dials the same number, but you modem answers each new incoming call on a separate connection. You need to purchase as many connections as you have customers. The modem routes all the dialup users to a PC, and the PC gives out IP addresses by DHCP, and you need to purchase IP addresses from a bigger ISP. The bigger ISP will do everything else for you. All you need to do is purchase an IP and subnet, or subnets. You configure DHCP on the linux router to only give IP addresses in your IP and subnet.
You can't take a single phone line, and a single broadband connection, with a single IP address, and make an ISP out of it. You need a phone line for each customer, and an IP address for each customer. IP addresses are not made from thin air. Every IP address on the whole intenet is kept track of by DNS servers. Every ISP purchases an IP and subnet from a bigger ISP. Then, the smaller ISP sells the individual IP's to dialup customers.
Designing and building your own ecommerce site is very difficult. It is better to have a web host do this for you, as they have all the software already configured, and they just need to customize it for you. They will register a domain for you with one of their IP's. You buy a domain from one of the many domain registration sites. You can check which domains are available, also. For the money you collect on dialup, it is definitely hard earned money, but it's honest, and I compliment you for your efforts.