That seems pretty easy enough, I never really had it explained like that. I am considering attempting to buy and sell websites, and using a reseller account seems easy enough for me. Since I have never used a reseller, is there a limit to how many accounts you can make? And could I use my main account (with WHM) as a website also?
There's always going to be a limit on the amount of sites you can host. The host might not limit you but there's always going to be a limit to the amount of sites hardware can handle. Even if the hosting company allows "Unlimited Sites" you have to take into account server resources like CPU and Memory usage. If you start hosting unlimited sites on a basic plan it's just a matter of time before their admins either disable your account or start charging you more money for excessive resource usage.
Hosting 25 sites on a basic reseller account is going to cause a lot less load than hosting 2500 sites. Typically the more sites you host, the higher package you're going to need to purchased due the CPU and memory usage more sites would require.
If you ask some of the sales people at some of the larger hosts, they'll flat out lie to you and tell you it's unlimited. About a month ago I had a HostGator Sales person tell me I could host 5,000+ sites on the basic reseller account which isn't even possible.... well possible but highly unlikely a server is going to be able to handle 5,000 sites on it. Hosting 5,000 sites would require about 12 servers and with them only charging $24.95/month for their basic basic package they'd lose about -$2500 per month on just the 1 reseller account. Hosting companies will tell you just about anything to get you to sign up and then restrict your account once you're overloading their server.
The rule of thumb we tell our resellers is about a $1 per site. If your plan is $25/month then you can host about 25 sites. $100/month you can host about 100 sites. These are not set limitations but they are limitations the reseller should keep in mind if they want to protect the performance of the server and their customers' sites.
Remember it's unlimited sites, but it's not unlimited CPU and memory usage. The more sites you host the more CPU and memory they will be using.... the higher the package you will need to upgrade to.
Also yes you can use your main account on your WHM account.