All of my themes and plugins are sold under the GPL, which doesn’t restrict your usage in any way. You are free to use the downloaded software you purchase on any website.
However, your purchase gives you a license key which entitles you to support and updates for your product on one site for one year.
If you want to continue receiving updates and support, you will need to renew the license each year.
Yearly license renewals are an important part of my business model. Recurring revenue from older products cuts down on the pressure to continually release new products and allows me to invest in the longevity of my existing products.
Please get in touch if you have any questions about my licensing policy.