Selfishness is one of my least favorite vices. Stuff is just stuff. Things are just things. I remember reading in Randy Pauch's, Last Lecture that he never wanted the people he loved to ever think that things mattered more than people. I wholeheartedly agree.
So, after I had two boys who wanted the same things very often and who often laid claim to ownership, I realized that I hated spending every mothering moment deciding on who owned what. It drove me crazy. So, we instituted a rule in our house: On Christmas and on your birthday, YOU own your items. You can decide if you want them the entire day or want to let others play, but everyone has a birthday and everyone gets new stuff for Christmas (and everyone remembers how you acted). Every other day, the toys or other items belong to our family as a whole and anyone is allowed to play with them, as long as they are respectful, and we switch off reasonably (with a timer when they were little, now they get it without a lot of monitoring).
For the most part, it's worked amazingly well for lots of years, especially since we have 3 boys all so close. I spend very little time regulating and I really like the way my boys, usually, kindly allow their brothers, friends and neighbors to use their stuff and feel comfortable sharing what they have. There are a few exceptions to the rule...bikes are the owners (espeically since everyone has one) and if everyone has something new that is the same and you loose or ruin yours, you're kind of out of luck for a bit.
For us, it's led to a pretty harmonious home regarding toys and other things and this feeling has extended to a more general attitude in life...my boys pretty much know the expectation is to share, even treats after sports games or bites of ice cream with their mom. That's just how we roll.
There are some drawbacks...there is less sense of ownership, so sometimes things aren't taken care of as well as I'd like, once in awhile it may be better not to share (food allergies, etc.) so we have to be careful, and they often assume everyone is as happy to share as they are and that is not always the case, so we have to talk about boundaries and respecting other peoples' stuff, but I feel like, for the most part it has worked really well for our family, especially with 3 so close.
Now, with this new little one, we'll see how it goes. Five years is a big age difference, but I'm hoping to somehow still create that same attitude in him!