I bought one from ebay as well. I think its the best thing ever.
Try putting it just above your pubic line that where i found my babys heartbeat. First heard it at 11 wks when we recieved it. Listen to it most nights as well.
Gets a bit scarey sometimes as we can hear two heartbeats, plus my heartbeat. Myheartbeat is obvously slower but the other two are really fast, we just keep telling ourselves no theres only one in there all the scans have said so, so far.
I reckon it is mine but hey you never know. LOL