It is rather rude to make a completed deal except for the exchange of addresses and then juset simply not respond again. Were it just one person, I'd say it could be any number of things. In fact I had one once who I had the same thing happen with, except that a couple of weeks later the pins we had agreed upon showed up in my mail. Seems they had just forgotten to send their address and I thought they had decided to say no. Good thing was that I still had the ones they wanted and sent them as now I had their return address on the LBE lol. Emailed them anyway and explained and they were a bit red-faced and understanding that theirs would be late.
The worst type to me are the ones who send you a TA and when you respond that yes, you would love to make the trade, they act as if you sent them the request and were insulting them with it. Apparently, accepting a trade too fast or with too much enthusiasm gives some the idea that their pin was worth more than they thought, and that it couldn't possibly be a good deal for them if accepted. Don't go looking for prices on everything, have an idea what you like and what you would give up to get it and if it fits in these criteria, then go for it. Spend all day searching out the values of pins, and eventually that's all you will be doing instead of trading, lol. You win some, you lose some, that's how it works, but in the end it usually balances out.
As for those who want to instantly renegotiate an accpetance of their offer, those are the type who deserve no response, but I still say thanks but no thanks. I have no problem accepting a no if I happen to go overboard, so I don't have a problem saying no either. You make a deal, great; you don't make a deal, move on to the next one. It's not personal, it's just trading!