A home is an investment, for the most part. Lots of people were just buying them for rentals and to re-sell at a much higher price later. I don't know anyone that's buying pins right now just for the hopes of re-selling at a higher price later.
Pins are high right now because new people have entered our hobby and they are impatient.. they want the best titles NOW and are willing to pay the high price. I've waited a year or two to buy some titles, and happily did it. Some of the new guys are just throwing the ridiculous cash out so they can have instant gratification. Sellers have noticed this and they are thinking "hell, if someone will give me "X" for this pin, which is way more than I paid for it, then I'll sell it. Buyers are putting out the big cash. I kind of think it's funny when I see a newb declare that they have been looking for one of the top titles "for a whole month!". They want it now, and have driven the market up because of that.
I think the market will level off, mainly because we have all kinds of new people joining the hobby and the demand will always be there. The new Sterns and JJP's are very nice, but there is no substitute for the classics- they are timeless and have stood the test of time. I don't see a "bubble" bursting. I think this is the guys that want the top titles dreaming about everyone else that has them losing their butt. I haven't been able to afford the big titles, and probably won't be able to at todays prices, but I'm realistic that they aren't going to get cheap in the future. I'm lucky enough, though, that had several pins in my collection that I bought years ago that have gone up, so trade value is what has saved me.