If you're doing a pay-to-play model, you might have it such that items acquired as loot are subject to being lost -- perhaps after a certain number of deaths, or on a dice-roll during death -- but that items bought through a real-money transaction wouldn't be subject to such loss. You could also augment this with being able to enchant looted items for a smaller fee, such that they are treated in the same way as paid items. Further, paid/enchanted items could either be permanently safe, or just have a really high resistance from being lost.
I'm not a huge fan of pay to play if it means buying an advantage over other players, but it seems to me that successful $$$-to-Play games that are enjoyed by all types of players are those that are selling convenience and time-savings, rather than pure advantage, and that the model I described above would fit into that category.