Purchasing from private sellers record keeping

[ukpartydj]

I'm sorting out my books for last month and I realised forgot to get some sort of receipt from a private seller off gumtree for a PC monitor.
I've only had this happen to me once before and I simply popped a "receipt not provided, this was the amount paid cash and place I bought it" into my receipt folder but was wondering if that is even allowed? / what other people do if it ever happens to them.

[chrisj]

Do you have any emails with reference the purchase as if so you may be able to use one of them if it has the cost on it?

[ukpartydj]

Do you have any emails with reference the purchase as if so you may be able to use one of them if it has the cost on it?

Not for this particular one, it was literally I called them and turned up and bought it.