I shoot left handed and own two ARs, a PC S&W M&P15 and a Sig 716. I've never encountered any inconvenience in firing either of them. In fact, I've never encountered any inconvenience in any semi auto rifle I've owned except the Browning bottom eject 22. The way I fire in the standing position, the 22 cases would eject onto my (bare, in the summer) forearm.
I would only buy right handed ARs as resale would be much easier and more profitable.
"I say, boy, I say, you're doing a lot of choppin', but no chips are flyin'."