I don't know of any state that its illegal to sell used car seats. Some shops won't because its a huge risk. Our big shop here actually puts the straps in the washer! So if it was safe before it sure isn't after they clean it. Who knows what she uses on the shell. I got kicked out last summer because I questioned all the expired seats she had. She doesn't believe they expire. It's just to get people to buy more.
I've met many people who don't get the whole "expired" thing. But kudos to the Graco company. I recently had to replace DS's car seat, and bought a Graco Nautilus. Clearly imprinted in the plastic in 2 places on the seat is an expiration warning, with the words, "Do Not Use Seat after xx-xx-xxxx".