COMC - Almost all cards stored on-site - Sellers decide their prices - COMC responsible for shipping cards to customer, customer can buy cards and leave them there to sell later
Sportlots - All cards kept at seller's premises - Sellers decide their prices - Sellers responsible for sending caards to either customers or to a box at Sportlots (relatively new feature) - Only then is Sportlots responsible for shipping to customer - Customner cannot leave cards there to sell later
Minimum pricing CAN be deceiving. 20 cents at Sportlots and 40ish cents at COMC, however, any order sent from COMC has 1 shipping price (depending on the level of service you request) no matter the number of cards. Orders from Sportlots can be from multiple sellers, but then you're also paying for shipping from multiple origins, not one like COMC.
If using COMC, the best bet is to open an account and add money to it. Having an account gets you better list pricing and opens the door for price negotiations (with sellers who allow offers). You can enter in 10 cents on a card offer (below the minimum price for any card), and COMC will tell you if the seller is accepting offers and what the minimum offer is (some sellers don't like receiving the minimum offer and may reject it). But it can get you better deals.
If using Sportlots, try to limit your orders to as few sellers as possible. Also, check the seller's shipping chart for prices and thresholds. If you have 8 cards from one seller, see if they have 2 more cards you want - the postage won't go up (tiers are done there by 1 card, 2-4 cards, 5-10, 11-20, etc.). Also, some will automatically jump to "premium" shipping if your cards hit a certain price point. If you decide to try the "box shipping", try to find the sellers that charge very little (or even nothing) for that service. Remember, you WILL be paying Sportlots for the "box" shipment to come to you. (And if they overcharge, they will issue a credit. I did when I tried this.)
If you have any questions, feel free to contact me directly. I have been using both sites for about 8-10 years now.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).