do 2:1 trades with people who are behind, those will use those extra cards to screw someone else, those who were going to do a 1:1 trade with someone else to get those resources, those who were not going to need to do port or bank trades to get those resources, those whose turn it just was and therefore have to wait the longest (see 2 above) and have a good chance to get those resources you gave them anyway, those who already have a lot of cards and may attract the robber because of your trade, and of course those who are not just going to use those cards 