I have 2 daughters 9 and 6, they have been converted to the lego affliction!!! I just spent about 8 hrs yesterday building friends heartlake shopping mall, and Regana's castle, trying desperately to watch the footie at the same time! We do buy some sets new if they are on offer, but my kids have been quick to realise they can usually get 2 sets for the price of one if they get them 2nd hand. I only buy sets that have the booklets and are 99% or above complete. Touch-wood everything I've had from ebay is perfect, just keep a small paint brush hand to dust the pieces as you build!
The problem I have now is that the kids have me searching stuff on the bay, and I am seeing all the super expensive starwars sets, especially the UCS, making my fingers twitch. I'd have to buy two of each though, one to build and the kids to play with, the other to store in the loft.
We don't keep the boxes, but I file the instruction manuals away after building, no doubt in the future once the kids get older, it'll be fun trying to remake the sets and sell on, it's kind of an investment, it never loses huge value even having been played with, as long as it's not too faded and is well looked after.