This is simply for your own protection. The most secure way to communicate about Reverb transactions is via Reverb's messaging system, and the most secure way to submit and accept payment is via Reverb's shopping cart / checkout system.
Reverb specializes in removing the risk from buying and selling gear online, and the only way we can provide the best service in the event any problems arise regarding an order is to have a record of all communications between buyer and seller.
Not only does this ensure your order is totally protected, but it also helps us keep the lights on, keep our fees extremely low compared to our competitors, and keep this amazing community of musicians thriving.
If for any reason you need to send your direct contact information to another Reverb user, just contact our support team and we'd be happy to assist if possible.