I find that the older I get the more 'sales-resistant' I get. Maybe it's because I'm running out of room to store everything. :D
You can't realize you don't need it, or won't use it, until you've bought something like it in the past, or as you say, you run out of room and think "do I really need something else here taking up space?"
The paradox is that I am now buying less as my income increases. The good thing is it puts me in a better position to buy a house, start a family, etc.
Heh, I try not to make the same mistake twice!
There must be some website where users post pictures and descriptions of ridiculous things they've bought and the money they've spent. How about calling the website: ThinkTwiceAboutThePrice.com or IOughtNotToBought.com or BuyersRemorse.com. Then with enough membership you can turn it into a swap/auction site. :D