IMHO
Sportslots is for selling commons which sell for $0.05 to $0.50 a card.
COMC is for selling inserts, SPs, low-end GU which sell for $0.50 to $10 a card, or possibly higher.
eBay is for selling high-end, or one-of-a-kind items.
I personally sell on COMC and eBay. Anything not worth...