DC 5V to 3.3V DC-DC Step-Down issue
I found the datasheet for the device used on your board here.

First, this is not a buck regulator like the E-Bay seller is advertising it as. It is a low-dropout linear regulator. Luckily, for the input-output combination you're using, that probably does not cause too big a loss of efficiency. Your efficiency will be about 66%, where an actual buck regulator could get 80-90% at the expense of more components on the board (higher cost).

Second, be aware of one other issue that could cause the behavior you saw. This regulator has a minimum load current of 10 mA. If your load (plus the LED onboard the part you bought) draw less than 10 mA, the output voltage could also drift up above the normal operating point.