Even if you get back with a "hey I'm busy and will look in a few days" that is better than complete ignoring. The relevant portion of the Constitution cited below.
Off Topic
Trade Talk Interaction
Members are expected to respond politely to fellow GMs trade proposals (no matter how unrealistic you may find them to be). This can be as simple as saying, “I am not interested at this time.” Members are expected to respond to trade communications – however briefly – within two to three days of their receipt. If you have sent a communication to a fellow GM and received no response within this time frame, you may send a complaint to the Commissioner.
Members are expected to respond politely to fellow GMs trade proposals (no matter how unrealistic you may find them to be). This can be as simple as saying, “I am not interested at this time.” Members are expected to respond to trade communications – however briefly – within two to three days of their receipt. If you have sent a communication to a fellow GM and received no response within this time frame, you may send a complaint to the Commissioner.