Yes, you will generally get less for direct buy out up front. Like
@lulilu said, there is more risk with consignment and you will wait much longer for payment. There is always the chance that the item won't sell right away and will go through markdowns on consignment and you'll end up with less than what you would have with direct buy out. I have done both, direct buy out and consigned and I prefer direct buy out generally.
Honestly, with a black caviar Chanel classic flap, it's a highly desirable item but I'd still submit it to 3 or 4 places for quotes. TRR and Yoogi's and Ann's Fabulous Finds will give you options (a direct buy out price or what they would list on consignment).
Fashionphile will provide you with a direct buy out offer. Then you can compare and contrast. Ann's Fabulous Finds will likely provide one of the higher estimates for consignment but I've found that items there tend to linger. Still, a Chanel caviar flap might go quick.