Why would you have that many PMs with so few trades? PMs are for exchanging of addresses, not for making trading details.
pm 1: email saying are these the details, can we confirm addresses (after all, there could still be a misunderstanding at that point)
pm 2: yes true, sometimes they do not leave the address right then so that adds a potential extra pm (even more potential extras, they decide they need to redo part of the trade and you have to go back to the thread)
pm 3: my address
pm 4: they might say they shipped it (probably half the time)
pm 5: every time: I say I received it so that they do not have to worry, and then leave feedback
pm 6: when you say you received it, they usually say glad to hear it et cetera
pm 7: (at least half the time) a pm to say they received it
it adds up fast, and that is without any complications (I did not count the potential extras after all), and aborted trades are also always a possibility
about half the time a user fails to reply with the original message train, it gets even harder to manage the overall details of the trade
when I have to go and figure out what to delete before enough trades have completed for me to save the space, I do it, but it should not be something anyone ever has to consider...if there is some good reason not to allow for extra space, all right, but nobody is really complaining without due cause, I mean on a normal forum 50 pms is like what you get punished with for infractions