Hi there

I have recently bought a cheap heart rate monitor/cycle computer from eBay (oh no, sounds bad already doesn't it?) and I reckon the thing is wildly out. If I measure my resting pulse it's about 60bpm but the HRM says it's about 90 and bounces around quite a lot. The seller is telling me there is nothing wrong with it and in fact you can't measure using a watch and a fingers technique, that the device is far more accurate. I wonder if anyone with experience of these devices could help me out?