I am an atheist, and I have had the misfortune of being evangelized to at the registers.
Here's a selection of what has actually happened to me:
1. They freak the fuck out and start babbling about their imaginary friend's super powers and demons and all the torture I'm going to get for eternity in hell because their imaginary friend "loves" me so much.
2. Passive-Aggressive Condescension: i.e. "I'll pray for you," (subtext: "...to have my personal religion's values imposed upon you, for your own good, against your will,") or "I'm sorry to hear that" (subtext: "... you will suffer in hell for eternity for not believing in my God.")
or, my personal favorite:
3. *Sad face* "Why?"
... Will you please just buy your items and get out of the store? There's like, ten people behind you that don't want you holding them up.