Not sure about the issue of prices being fixed. I've checked a number of parts at both JDParts for pick up at the local dealer and some of the online sources and the online sources are less. Looks to be about 10% less for the items I checked.
I don't exactly how the parts cost, shipping, tax works out, but it's probably close either way. I personally like the convenience of working up my parts list, sitting down for a short time at the PC after dinner one night to place my order and having the parts show up at my door. Plus, I'm not in love my local JD dealer. I guess I was spoiled by my local NAPA auto parts back in my high school and college days working on cars. Great group of guys, friendly, always willing to help on parts, a little troubleshooting help if they could offer it as well. I don't get much help at all from local JD dealer, except removing money from my wallet.