Template or theme — the distinction that matters later
A template is typically a set of HTML, CSS and image files: you own them outright, they never update themselves, and nothing breaks when a platform changes. A theme is a template packaged for a specific CMS — WordPress most often — which means it plugs into that system's hooks, its plugin ecosystem, and its release cycle. The convenience is real; so is the coupling.
That coupling is why buying a theme is a maintenance decision wearing a design decision's price tag. A theme has to keep working across years of core updates, PHP version bumps and plugin conflicts. If its author stops shipping updates, you have not bought a design — you have adopted an orphan, and the eventual migration costs many multiples of the purchase price.
The multipurpose trap
Best-selling themes usually do everything, which means they load the machinery for everything whether you use it or not. A portfolio site running a theme that also ships a booking engine, a forum and two page builders is paying for all of it in load time on every single visit. Narrow beats popular more often than the sales rank suggests.