In terms of alternative brew devices that you might stock in your store in addition to a french press, I'd say the Aeropress and a straight-up drip cone.
The Aeropress is kind of a love-hate thing for me. I find my results very inconsistent... sometimes awesome, sometimes disappointing. I rarely reach for it any more for that reason. But, like the moka pot, it will do a coffee concentrate for a latte-like drink. They also sell well, in my experience. We carry them, get lots of questions, and it seems like every demo sells a unit. We've now run out at the shop and are reordering.
To me, you really can't beat the drip cone. Not a real moneymaker for you, though I suppose you could stock one of the Bodum ones - like the Kona or Bistro Drippers, or the ceramic ones at a higher price point. This or french press is the way I brew now when I really want it to be good. A real attractive aspect to this as a retailer is that you can pair it with a $25 whirley-blade grinder (and paper filters) and make an ass-kicking cup for about 30 bucks investment.
Honestly, I have very little love for the moka pot. Maybe I'm just not using it correctly, but I have difficulty getting good results. Yeah they're traditional, sure they look nifty, but of all my brew apparati this gets the least use...
My 2 cents.