The two independent stores near us both do this, but they have carseat techs who come in on Saturdays to do it. (They also do inspections, and I think both are free---the inspections for sure are, and they will inspect seats even if you didn't buy it there; not sure on the installation as we're planning to install ours ourselves. They won't sell the seat we picked till the fall, so we had to buy it elsewhere, unfortunately.) It hadn't occurred to me that there might be liability issues, though---oy! Maybe we should just go to the state police check, which is equally near our house...
ETA: Just checked the website of the store we're registered at. Installation is free if you bought the seat there, but there's a $25 fee for installation/inspection if you bought it elsewhere. I had in my head that the inspection was free either way....oh well! It does say certified child safety seat specialist, though.