The reason he's nervous is because he's selling accounts which potentially could get banned during the sale. Clearly he requested that you wait to protect both himself as the seller and you as the buyer from this kind of situation. If the account he sells you is fine after the cooldown time doesn't need to refund as clearly the buyer has got it banned. However if it's banned after the cooldown then he has to refund the buyer. Henceforth I wouldn't exactly call it nerves, he's just being safe.
If you can provide evidence that you didn't bot on it after your purchase then let us know.