While there are a couple of places online where you can sell vst plugins, those places are often a little unsuitable since they were origianlly designed as forums. Instead filterbank.net was coded from the ground up to try to solve a lot of the problems when buying and selling plugins. For example you may ask:
Who am I buying from / selling to?
Are they reputable?
What's the best way to do the license transfer?
How do I transfer the digital license?
Is the company that created the plugin ok with me transferring the license in first place?
This website tries to address as many of those concerns as possible. And in doing so strives to create an easy, safe, legal and efficient way to buy and sell second-hand plugins.
So basically this website will just inform and connect buyers with sellers. After that the buyer and seller complete the transfer outside of this website. Then they rate each other to build up a reputation here so that a community of trustworthy buyers and sellers emerge.