The most important thing is to set geodesic to true. This will create an accurate buffer from projected units and the bufferSR parameter will be ignored.
If you have Web Mercator input geometries, the inSR and outSR should both be set to 102100. In addition, make sure that you specify the buffer unit correctly. The US survey foot has a WKID of 9003 and the international foot is 9002.