For what it is worth...
Picking live rock from a LFS is really a matter of personal opinion - I've found rocks that I love , built a structure that I think is awesome...and then take six months to sell. On the other hand, I've grabbed "scrap" rock out of one of my bins and tossed together a 'scape just for fun and have 10 requests to buy 10 minutes after I post a pic. Moral of the story? Look for rock that appeals to you.
Have an idea of what you want your 'scape to look like. Caves take different rock than bonsai, bonsai takes different rock than overhangs. I prefer to get rock form a single source on a build if possible, it adds continuity to the 'scape. Make sure to know your tank's measurements. Most people overestimate how much rock they need, then since they have it on hand will force it into the 'scape so they don't waste it. The old 1.5 pounds per gallon is the perfect recipe for a rock wall - which is great if that is what you are looking for...otherwise shoot for trying to figure out how the rock will go together in your tank before you buy.
Make sure to find out if the rock is cured - sometimes the LFS gets in a fresh batch of rock which will take longer to cure. Not a problem, just be prepared. I am a huge fan of Real Reef, it is man made so doesn't impact the wild reefs and is pest free and ready to start a tank. I have grown to prefer using dry rock over live as well, no pests and products like Marco Rock are also not impacting the reefs - you can do amazing stuff with that rock! As for seeding a dry rock build? I recommend Dr. Tim's One and Only. I've been in the hobby a long time and this is the only seeding product I recommend - I've 'scaped dozens of tanks that were then started & seeded with Dr. Tim's - they work every time.
Look around to see what you like and if possible ask the tank owner what type of rock they used...
I'm obviously a fan of assembling structures using Emaco over stacking, but that is another question....