Put it right after the trade offer appears on screen, then follow it with Nope Meme Sound to make the refusal clear, or with Okay Let's Go Meme when the deal is accepted despite the warning.
About this sound
This is a villager's brief nasal hum from Minecraft, lasting just over a second, a low, gruff 'hmm' that lifts a little at the end. It feels dry and unimpressed, like a quiet question or a polite refusal to agree. It comes from Minecraft, the Mojang sandbox game first released in 2011, where villagers make this sound when players approach them to trade.