1. Don't lose your cool or it will not work out well.
2. Keep on him with an email every day until he responds
3. If he doesn't respond in a week or so, report him to eBay. and see what THEY can do about him. One bad feedback is not career threatening, but if you report him to eBay and THEY take action, they could suspend him and make him start from scratch. THAT is career-threatening.
There are people who claim their items are near mint and work 100% just by looking at them. I bought a calc from just such a guy. (A Canon 5-function Palmtronic) The damn thing had exploded batteries in the compartment. Even after I cleaned all that crap up, it didn't work. I didn't see any way to get it apart without making it obvious I had it apart, so I just complained to the guy. I asked him how he could guaruntee that it works '100%' without even turning the !@#$% thing on??!! The guy was so rude it was unbelievable. He explained that he got these things by the hundred and just to send it back and he would refund my money. The guy couldn't be bothered to actually take 1 minute to check the item he was selling. He also told me not to bid on any of his auctions in the future. Apparently, I was a bad customer because I complained about an item that didn't work. He also waited a week after receiving payment to ship.
All through his insults and misbehavior, I kept my cool. My plan was to keep my cool until I got my money back, then give him the negative feedback. But he wrote me an email before he sent the money back apolgizing for his rudeness and offered to keep his eyes peeled for a calculator I wanted. Incidentally, he is also one of those sellers that doesn't give feedback to his buyers until they give him feedback. So I decided I would wait 89.9 days before leaving his feedback to avoid the retaliatory feedback. But I softened up a bit after his apology. Then a week later, he left positive feedback for me even though I was "obsessive-compulsive about my $10 item" So I figured he did stand by his guaruntee, and although that doesn't excuse his rudeness, I would overlook that small detail.
So play it cool and follow the proper channels. If that doesn't work, (give eBay time to respond) leave him a negative feedback, but with no trace of anger or malice. You lose credibility that way. If you just state facts, people will see that you're not angry and will be much more likely to believe you.
If worst comes to worst, I bet it wouldn't be too bad to fix. If you search the HP48 sites, there is one with directions on how to disassemble a 48G (basically the same as yours) without any outward evidence of having done so. It is probably just a matter of taking it apart and cleaning the Coke off of the contacts with alcohol.
If it comes to that point, let me know and I will do my best to find that link for you.
-Jeremy
P.S. - I will take this opportunity to gripe again that HPs are no longer put together with screws... :(