If I made a game using polycode, could I copyright and/or and sell it?
Polycode is licensed under the MIT License - it allows you to do mainly whatever you want to do with our code - you can copyright everything you do with Polycode. For further information maybe have a look at the actual license text or for a good summary see: https://tldrlegal.com/license/mit-license
Well - as I understand you would have to mention somewhere that it uses Polycode - but as long as you do not copy the source code along with your project you should be fine not to provide a copy of the MIT license..
(Disclaimer: I'm not the copyright holder or a lawyer :$)