In Catan, can players negotiate a trade when it's not their turn?
Catan is a social game where a degree of social banter of the sort above can enhance the feel of the game.

However, considering that the game permits trades with the active player - and not between non-active players, it is plausible to rule that such is interference with the active player's turn and therefore not allowed. 

Even in larger games with 5+ players the game rules do not cater for trades by inactive players where the active player is not involved - and it is considered etiquette to process conflicting build requests in turn-order for precedence (hence no non-active player building a town on a contested space that is being built on by the active player).

What level of trade banter you permit is down to that agreed. We have played fine enough with some banter but it can get annoying when excessive.