The merchants in the Battlefields change their inventory. When I re-entered the BF they had new stuff - sometimes even during the time I was there. Try hopping in and out of the BF and check the traders then <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />
No, you can't store anything for keeps out of your inventory. As soon as you change the act, the stuff in the BF is gone, as they are generated anew as soon as you enter the new act. You can store stuff within an act => at the BF.
what about this: you store suff, until you're nearly at the end of the act, pick the stuff up then and place it into the new act? Though, items requiring your chars to be >lvl 40 will not be of much use. I didn't level up in the BF and was lvl 39 at the end of the game.
Kiya