Translation:I want to buy the watches in cash.
In English you buy things FOR cash, not IN cash.
As a native English speaker, I would never buy things for cash. I'd "pay for things in cash", "buy things with cash", "buy things in cash", even "sell things for cash". Lots of prepositions work here.
Mostly "buy with", but also "buy for", "pay in", but almost never "buy in".
This should be "I want to pay for the watches in cash."
"Quiero comprar los relojes en efectivo."
translates literally into English as 'I want to buy the watches in cash.' This is incorrect English idiom. In English we buy FOR cash or pay IN cash but we do not buy in cash.
'I want to buy the watches for cash' was rejected in favour of 'in cash.' The former is correct English, the latter is not.
It should be to buy the watches for cash. That at least is how American speakers would use this.