1) When you and the other party have agreed upon a price and exchange (screenshot this), you can ask any of the moderators who are online and available at the time to chaperone the transaction. If none are chatting in the in game chat, please check PL's IRC channel- this is where many (but not all) moderators idle while they are online.
2) The buyer sends the payment to the moderator to hold onto. The moderator will confirm with the seller (via screenshotting the offer set to their trade and/or linking the ID of the Pokemon if a specific Pokemon was agreed upon) that it is the agreed upon price and/or Pokemon.
3) The seller then donates to the buyer. The buyer must confirm to the moderator that they received the donation (if the buyer lies about receiving the donation there will be dire consequences for this).
4) When the transaction is complete the moderator forwards the buyer's payment to the seller.
Answers to possible questions you may have about this: