As far as safety and simplicity go, Playerauctions is by far the best. If you order with the top rated people there it is a breeze. You'll get your order in less than 20 minutes after your credit card or other payment option go through and that's the end of it. On Sythe, you have to make a post or a PM to the person you wish to buy from. It takes upwards of 24 to 48 hrs for them to respond which makes the coordination effort of getting you the gold difficult. Also, the payment process is always odd as you have to make a donation to another persons paypal account or use some BS complicated service like western union (I.E. wiring money).
TL;DR: Playerauctions FTW.