I'm closing in on 10,000 cards sold on COMC, so I know a bit about how it works.
The first thing you should know, is that cards don't sell instantly. Some people have the expectation that if they send in their cards they should sell through in about a month. For my port size, I sell through well above average. My port is usually 2,000-2,500 cards and I will average around 20% sell through in a month (500 cards).
First step is to figure out what to send in. As mentioned, don't try sending in cards that have fifty copies on the site...usually. Try and find cards that are unique, but also cards that should sell. The only base I will ever send in is major star stuff and rookie cards. Even then, I try and limit what I do send in. When you add funds to the site, you should receive history points. You can also earn these through the COMC challenge. Use the history points to determine how well a card has done in the past. Sometimes there are cards with a population of 50 on the site...if you check the sales history, you'll find that over 500 have sold and shipped and that this card continues to sell through on a pretty regular basis (Bryce Harper Rookies, Puig Cards, Trout Cards, etc.). My rule of thumb is to not send in anything I can't list for at least $1.00. I figure for the effort involved and the fact that you need to leave room for price drops, that is the threshold most of the time. I do send in cards that sell for less than $1.00 occasionally, but they are major star issues and I usually use them to finish filling a box I am ready to send in.
When you decide what to send, the next step is to ship to COMC. Pretty straightforward process that they will walk you through on the site or over the phone if you need help (the CS guys are terrific). When they receive the package, the clock starts. Cards should start to drop after 2-3 weeks, lately the majority are falling between 3-6 weeks. You will usually have the last few stragglers make it in right before the 8 week deadline.
Each card submitted costs .25 for the basic service - right now they are not charging you until the card is processed (it used to be all up front). Unfortunately you just missed out on the National Special (550 ct box for $77.00). As mentioned, they will scan and list the card - you then price it. When it sells, they deposit the amount of the sale into your account. The buyer does pay a .25 built in shipping charge per card, however if they are in simplified mode it doesn't show as an addition. The other mode is advanced which is used for flipping cards on the site - you don't pay shipping on these cards unless you decide to have them mailed to you later. So, in simplified mode a card you list for .50 will show up as a .75 card. When it sells, you get .50 and the other quarter goes to shipping it.
If you choose to use the credit to buy on the site, there is no additional charge. If you decide to cash out the credit for Paypal, they take 20% of the credit as a fee for cashing out. You can also get certificates to Blowout Cards through the store credit at a slight discount over the cash out fee - something like a $250.00 certificate costs $290.00 which works out to around 13-14%.
Some people gasp in horror at the 20% cash out, but if you do the math COMC is actually really reasonable for cards that are low end like this. Basically eBay takes the 10% FVF, Paypal takes 2.9% and .30 cents. On a low end transaction, that .30 raises the overall percentage significantly.
Example:
$1.00 sale on COMC - .20 cash out = .80 - .25 Listing - .55
$1.00 sale on COMC - .10 FVF = .90 - .33 to Paypal = .57 (if you had to pay .30 to list it you net .27)
For the extra .02 (or negative .28 depending) you do none of the work that you would do on eBay.
That's pretty much it I think - feel free to ask more questions or advice. Always happy to help.
