Is it possible to reference or modify game-play settings via a script?

In particular I am looking to write a script that will modify fBarterSellMult & fBarterBuyMult based on different conditions.

I can't seem to find any info on this.

It can be done, but requires NVSE. Here's a link to the FOSE documentation for http://fose.silverlock.org/fose_command_doc.html#SetNumericGameSetting. Not all of the commands from FOSE have made it over into the current NVSE beta yet, but setNumericIniSetting has, and the documentation/usage is the same.

I will check that out thank you

GMST's, when altered in game, will revert to their default values upon restarting the game. To ensure your changes are re-applied once per save load, until GetGameLoaded is fixed, you can use a http://geck.gamesas.com/index.php/Globals 'bGetGameLoaded' global set to 1 and set it to 0 in GameMode right before all your SetNumericGameSetting/Con_SetGameSetting uses. Otherwise, you'll find the alterations won't stick.