[Q] Volume- button broken topic

I just dropped my Z3C on the floor right on the back and the back screen cracked. - Now I have just realized that the volume minus button is broken too. If i touch the volume- button just the slightest, it senses it and turns down my volume on the phone, which means everytime i hold the phone with my right hand it turns down volume. Have anyone else tried this?

Also, how do I know if the ebay sellers stating that the parts are OEM are in fact OEM?

I have tried testing the waterproofness of the phone after the broken back, but I don't know if my result says it is or it isn't waterproof, when i push the screen it rises from around 1022 to max 1027 Millibar if i push hard.

Best regards
Marcus