You said you would send them a few thousand cards. You have to pay them 25 cents per card up front, so 4000 cards will cost you $1000 plus shipping.
It really can be a great deal because they do all of the work for you, if your cards sell. Many of the cards on COMC have been sitting there for 3-4 years. Many sellers won't accept offers, or they only accept offers of 5-10% off.
You will receive a lot of low ball offers, but if you want to move your cards quickly you may need to accept these offers.
And remember, if you want to cash out, COMC takes 20% off of the top.
Good luck. Hopefully, I will be sending you some low ball offers.