Rust:
Putting a wet rag and soaking the key in evaposust will remove the rust easily without damage- it is non-acidic.
Better and easier- Seeing how its just very little if it were mine I would just use some metal prep like "Kleen-Strip concrete and metal prep-
It has phosphoric acid some...
The seller is wrong and does not understand what they are selling. Likely a chinese sales agent at a computer.
Its funny you just posted this as I was checking lead terminals to replace the skinny brass ones on the truck I'm helping on in case we needed them. And people please watch out- there...