basic stuff really, but usually you only pay cash if you're picking it up, in which case you'll know the address, I've only ever exchanged cash for an amp once at the motorway services, driving away I realised I'd not asked for a receipt but I had bought it through ebay so I was sort of covered. I'm surprised how often I've sold stuff for cash and not been asked for a receipt (never) but, again, they were picking it up from my house.
edit, I do ask for a receipt for high value items just in case they turn round and say I've stolen it from them