Shipping it yourself to the buyers FFL will get you screwed. (assuming the buyers FFL will take receipt from a non-FFL.)
USPS won't take it from a private citizen, and UPS/FX will want upwards of $70 or more, especially if it's a pistol.
Dealers however can ship USPS and depending how far, rates are around the $20. Priority mail, too, so it gets there pretty quickly. So even if you have pay your FFL $30 + the ~$20 shipping you're ahead of the game shipping yourself.