Nice score! I've been doing this for a little while now and can help a little. Be very careful. 50 year old plastic that has been exposed to light and heat can be very fragile and brittle. I cracked a sign just turning it over while cleaning it. I use Dawn dish soap and hot water to disolve the dust, dirt and nicotine. Be careful, anything stronger can stain the plastic or even destroy it. A magic erasure works good too. My wife says that I wash my signs more carefully than I used to wash our babies! As far as websites, I don't know of any but I'm on several Facebook pages where people buy, sell and share old beer stuff. I get lots of good tips and pointers there.