First off, sounds like you've been lucky with the phone for some time. I myself ride, and always carry my phone inside my leathers. One reason is I can't hear it/feel it any other place, second is my bike vibrates so bad that I'm afraid that I'll lose it, and lastly, as you've stated, I don't want to get it wet.
Now for the online question. There really isn't any way of knowing for sure whether or not the phone is going to work. If buying off ebay, look at the seller's rep, if it's up there, you probably shouldn't have any problem. But, buying off line is always a gamble. I personally have never had a problem with anything I have bought from there. It's just a chance your going to have to take.
If your that worried, buy a new one from your provider, that way you have no worries.