If something is of high quality to begin with, is rare and is kept in pristine condition, it will hold its value. As AG become more mass marketed and more "plasticy" its items will tend to go down in value. The original Barbies are very valuable today.

I consider AG a toy first to be enjoyed now with my DD. I wouldn't collect it only to hold its value. We have seen how the retired Felicity items have not retained their value now that AG has reissued some of them.