Many market vendors and taxi drivers will attempt to charge foreigners more. Be knowledgeable and stand your ground. If you know the taxi is supposed to be $300 Guyanese dollars around Georgetown , have $300 ready and hand it to the driver, or ask for the exact amount of change.
Prices vary more from market to market. Shop around. It is possible to bargain, but not necessarily expected. Often vendors throw in an extra something if treated well.