With COMC you can purchase cards from multiple people and then when you have enough cards amassed and you would like them delivered to you you can lump them all up together and pay shipping once. Shipping can take a while sometimes though.
Also, as soon as you buy the card you can list it for a higher price and possibly flip it for profit.
I once had a Leaf Mario Lemiuex auto I bought for about $60 USD and had it sent to COMC (since I am in Canada and the seller was in USA, it was cheaper to send to my COMC address). I listed it for $200 USD and it sold. So I made close to $140 USD on a card that was never in my possession and took next to no work to sell it.