When signing up to Bluethumb, on the initial sign up page you will be asked to sign up as either a Collector (an art buying account) or as an artist (artist selling account).
Sometimes people miss this option and accidentally sign up as a Collector instead of being an Artist. If this happens don't panic, there is a simple way to switch your account over.
When logged into your account hover over the top right where it says "Hello YOUR NAME!" and you should see a drop down menu appear. From this menu you should see, as the 8th option from the top, "Become an Artist", click this.
You will then be taken to a page to create your artist profile. You can follow FAQ How do I sign up as an artist? for more detailed information on creating an artist account with us.