How do brokers take your miles? Do all currencies let account transfers?
with regards to UR, they allow transfers to airlines, and they also have an option to transfer to someone else's UR account, but by transfering to another persons UR account, you are asking for trouble. so when selling UR, make it clear to the broker that you only want the points transferred to an airline and NOT to someone else's UR account. and as i said, i did this with "sell miles now" in august and they were very easy to deal with about it.
Regarding AA or United miles, they just book a flight using your miles
I have no idea about MR, i have never sold MR to someone (i assume its the same as UR?)