You'd probably only be able to trade with other people who have the same pruning settings on their chains, but this is just a wild guess.
Interesting, but I "don't get it" for now. Could you clarify why you think the pruning settings should be similar or give me a hint where I could research?
If you prune to 550 and your trading partner prunes to 950, then she has a 400 block informational advantage over you. If she is able to introduce a serious network disruption (perhaps a hardfork attempt involving a lot of hashpower!?) during the trade she might be able to take your coins while giving nothing in return.
I would personally want to make sure that I'm not risking anything if some kind of serious instability arises, especially when trading larger amounts. So in many cases that means no pruning at all.
If you and your trading partner were the only two participants on the chain you were transacting upon, then it might be an issue, but if every other participant on that network has a copy of the blockchain, the person attempting to introduce the disruption or fork would only succeed in forking themselves off the network, so that attack vector wouldn't work. It would take a significant proportion of network participants to be colluding in the act (basically what tends to be referred to as a 51% attack) to mess with your transaction in such a manner. Otherwise no one could rely on SPV, where you don't keep a copy of the blockchain at all.