Audiogon recommends that both parties stay in constant communication during any agreed upon transaction. If the payment terms are "Buyer prepays," then the Seller should ship the item within 7 days of having received payment, and provide the shipment tracking number.
If you do not receive a tracking number within 10 days after confirmation of receipt of your payment, then you may consider filing a Dispute with Audiogon: Submit a Transaction Dispute. You should first try to contact the seller via telephone as well as email. If the Seller is not responding, or you are not confident in the Seller's responses, you may consider contacting law enforcement agencies, as well.
PLEASE NOTE: The time periods described above are for domestic transactions where both parties are in the same country. Also, there may be special circumstances that delay a transaction, but both parties should be communicating about the situation. It is vital that you try to email, as well as call, the other party before assuming the worst.