Selling on Swappa is safe, easy, and free. Swappa has no seller fees (unless you choose to upgrade your listing to be featured) but buyers pay a small sale fee when they buy. (And PayPal charges a fee for all payments.)
To keep Swappa safe we don't allow the sale of junk devices and we review and approve all listings before they can be bought. So all devices listed must meet quality criteria, including having a clean ESN. And we require all listings to have verification photos to prove device ownership. Until a listing is approved it will be pending review for a short time.
On Swappa we use PayPal for payments because they provide extensive buyer and seller protections. For maximum protections you can set your listing to require buyers to have a PayPal confirmed shipping address.