I don't know if your buyer is dodgy or not but what is coming across is they definitely don't sound as if they are committed or intending to buy.
The only thing I can see that you have sold recently is that $7500 item where you accepted best offer?
Forget trying to 'report' the buyer as such. You need to pay attention to padi's post.
Your main concern here is that you have 'sold' a really expensive item and the fees on that are going to be high and you'll be charged those fees unless you do something about this now.
What you need to do is open an ebay claim for unpaid item. read the link padi gave you and follow the instructions and follow up on your claim too. What should happen is that ebay will allow the buyer a few days in which to pay but if they don't (and I do not think they will pay) you then have to finalise it and ebay will refund the fee amount.
The buyer will also get an unpaid item strike and if they collect a couple of them they may find themselves blocked from buying by some sellers. That's a punishment of sorts.
It probably won't worry your 'buyer' in the least as they sound new anyway and not committed.
Your first priority is opening your ebay case to get those fees reimbursed. Don't worry about the other things.
