I tried out a couple of the HC-SR04s I have at hand and they don't work at 3.3V. These use a MAX232 to drive the transmit transducer and that's a 5 volt only part. There may well be other issues with the design at low voltage.
The sensors linked in the first post clearly use a different chipset so it's not impossible that they do work over the advertised 3-5.5 V range. That said, these sorts of eBay listings are notoriously unreliable. Of note, this design apparently is using an 8 pin microcontroller without an external crystal.