ID:2407722
 
(See the best response by LordAndrew.)
Can anyone point me to the legal page for the BYOND engine or tell me what licence it's under?
I can't seem to find it anywhere.
Is it a GNU variant requiring you to release the code, or can you sell closed source copies of games you make?

Best response
The only license of sorts that BYOND has is its terms of service, which defines what you can and cannot do with the software itself. As far as code goes, it's 100% yours and can be distributed and licensed under whatever you like. Libraries (unless otherwise specified) are released under a public domain sort-of thing and can also freely be used in your project.`