Check out /r/pen_swap

Their rules also apply here, viewable here:

https://www.reddit.com/r/Pen_Swap/wiki/rules

Read before participating; outlines post template, requirements, condition ratings, etc

List of past scammers to avoid:

https://universalscammerlist.com/search.php

Be sure everyone you transact with has commented publicly on the subreddit before you send them money or items!
Use ONLY PayPal Goods & Services for payment.