OK, that's a good start. That at least narrows it down to the mic, as you said. Likely your radio is just fine. As I said, the microphones take the beating, so this isn't to be unexpected. As you can imagine, the eBay seller telling you it worked before it shipped, and now it doesn't raises some red flags. Inquiring with the seller will just result with the same story, "it worked when it was here". You could try asking, but I doubt you'd get any help. Negative feedback might help others, but don't completely rule out damage in shipping.
Well, next thing to do is carefully take the microphone and plug apart and start checking for continuity between the pins on the connector and the inside of the mic. At this point, if you don't own a multimeter, it's time to get one. As a ham operator, you will need one, and it will become your new favorite tool. You don't need an expensive one, but beware of really cheap ones. I own everything from $300 Fluke brand meters down to one I picked up at Harbor Freight for $3.00. Unless you are really strapped for cash, don't waste your money on the $3.00 Harbor Freight unit. They do work, but will fall apart quickly unless you baby the heck out of them. A good one that I recently purchased to keep in the tool box in my truck was the VC-97 model that's sold by a number of sellers on e-Bay. At $30, it's one of the better ones I've seen for the price. For the average user, it will work just fine.
The next trick is learning how to use it. There are primers on line, so I won't cover it here. Don't expect the owners manuals to be much help.
Use the continuity tester setting to start checking between the pins on the mic connector and the inside of the mic. If they all check out and pass a visual inspection, then check the actual push to talk switch. This is often a little push button mounted on the circuit board. It may be hard to check as you may need a second pair of hands. Put the leads on the pins, there should be three. One of them will be the common. The other two will behave differently depending on the switch. Testing continuity between the common and one of the pins should show continuity when the PTT is not being pushed and the other one will be open. Performing the same test again while someone pushes the PTT button should show the opposite.
If it does't work that way, then likely that is your culprit. Looking at the wiring diagram for that microphone shows it's a very simple circuit. The push to talk line from the radio is just connected to the radio ground to put it into transmit mode. Replacing this button may be easy, finding the exact part will be a bit more work, but not impossible. Likely the little button, if it is like the ones I've replaced will be quite inexpensive, on the order of less than a buck. Down side is that you'll spend more on shipping or minimum orders to get the part. It may be unlikely that your local radio shack will have it in stock. If you have a really good electronics store in your area, they will be of more help.
As for finding a new one, good luck. Due to the failure prone nature of these, it can be difficult to find replacements. Often people just want a new one rather than fixing the one they have. You can start searching, but it's unlikely you will find a "new" one. Likely what you'll find, if you can find any, is used ones that may be in the same shape yours may be in. If you do find them, expect to pay a premium. I doubt you'll find one in good shape for $20.
