OBS is published under the GPLv2, which means that if you modify the source code and build a new version of it, you have to also release the source code of your modifications that comprise the binary that you're distributing. You are technically allowed to sell copies, but there is nothing stopping anyone from building the program from your source code and releasing builds for free, since you're required to release your source code. So it doesn't really makes sense to charge money for your build of OBS.