I have never heard of runescape accounts or gold, but that doesn't matter.
There's more to it than just some HTML template. It mainly comes down to provide a secure way for people to be able to buy and pay through the site; add an item to a basket, calculate prices, preview the basket, secure payment, generate invoices, notifications...
Usually, that's done with some kind of e-commerce software (eg
OScommerce) that runs on the server. That might be in any server-side programming language (PHP, ASP, Perl, ...), probably combined with some database (mySQL or other). You need at least have some understanding of how these languages work, as often you might have to configure the software to suit your needs.
I'm not so sure if you are up to that.