What does that RME number mean? Would appear to be a stock number to me.
I would also just add that $420 dollars for a NIB G17 is a ridiculous price, and usually when things are too good to be true, they are.
I wouldn't trust it.
As suggested getting personal contact info would be a good idea, and his willingness to provide such might help with that.
Another suggestion which I usually ask of noobs around here, is to ask the seller to initiate the transfer and provide you the reference number before you pay. At least this way you know he is legit and that the gun exists, before sending money. He doesn't ship anything until he recieves the cash, and it is relatively easy to reverse the transfer if you backed out anyways. Again his willingness to accomodate your concerns will help you decide.
In the end though buying privately online through the mail requires a huge amount of trust from the buyer. It sounds like you already don't trust it, so........