I have the 5906 as well as a number of other SW 9mm 3rd gen guns (5904, 6906, 6904, 3914) and have tried carrying the 5906 IWB. Besides being heavy it also has a long grip and tends to print too much for my comfort level. I have not tried a shoulder holster. It's a great duty, house or range gun. The price he's asking is about average assuming you wouldn't have to pay shipping and transfer fees as if you bought it off the internet.
The absolute best 9mm 3rd gen. SW for carry (in my opinion) is the 6904/6906 platform. You get 12+1 rounds, alloy frame and compact enough to hide easily. And they are usually available at a good price (I've gotten mine through Coles Distributing off Gunbroker). Many like the 3913/14 pistol as well- a slim, single stack 9mm but a bit too small for my large hands. Good luck.
best wishes- oldandslow
PS- working next door to a gun store can be a distraction and tough on the bank account. Wish I had one next door to my office.