As a season ticketholder, I can tell you it's much easier to sell tickets using SeatGeek instead of a direct sell. There's a "Sell on SeatGeek" option in my BDC account. I just click that, select the tickets I want to sell, then it transfers you to SeatGeek where I set a price. After that, I'm done. If they sell, SeatGeek submits a deposit to my bank a couple days after the event.
For direct sales, I have to spend time finding a buyer. Go back and forth negotiating a price. Then I have to get payment and transfer the tickets. I prefer to get payment first but some people are hesitant to pay before you send the tickets. So sometimes you sent one of the tickets first as a sign of good faith before they'll send the payment. Some people will only send half payment until you deliver the tickets then they send the other half. yada yada yada. It's a hassle I'd rather not deal with. I'll gladly surrender a few bucks to SeatGeek to avoid that hassle and to save time.