I'm assuming you're not getting any flash player running slowly errors, so I guess the loop is working fine, in which case all I can think of trying is (i'll use j instead of i so it doesn't go italic),

sum="";
for (j = 0; j < numAuctions; j++) {
addToString = Auction_URL[j] + "\n";
sum += addToString;
}

This probably won't work, but it could be worth at least trying