If you'd asked this in the middle of 2020 I would've teed off on them but that was then. I started using them in 2018 and as another mentioned it's a very user friendly site - especially for selling. They do all the work and you just set a price and adjust it whenever you want. You can then either use the proceeds to buy cards or cash out. You do have to be careful what you send though.
You need to get around $0.85 for a card not to lose money even at the cheapest rate. They charge $0.50 per card but you as the seller wear a $0.25 shipping 'tax' and then there is 5% sales commision and another 10% if you want to cash out. Given some cards drop in price, especially when they are newish ones, you need to give yourself a margin to work with. Of the 400 I just sent in some of them are selling for $0.60 which means they have dropped 60% since I sent them in and I will actually lose money on some cards. If I sell a card at $0.60 the real price is 0.35 given the shipping 'tax' and then they take 5% from the $0.35. That means I'm getting $0.33 back from a card I paid $0.50 to list. I would then lose another $0.033 when I cash out. Thankfully there's only a handful of those in this batch and it looks like they are averaging out at $10 per card which will be nice when it's all done. If you are lucky some of the cards you send in will actually rise in price between now and when you sell them.
If it weren't for comc I probably wouldn't be nearly as engaged with the hobby as I am today. I'm a big fan of theirs( other than a period in 2020 when they were doing my head in). They are back to their pre-covid standards. Good communications etc.
Fun fact( though not for me). The first consignment I ever did with comc I sent in about 40 worthless aussie rules cards as box fillers not thinking they would be processed. Very naive of me. Of course they processed them so I paid $10 for that( it was $0.25 back then) and then just donated the cards.
In short make sure the cards are at least around the $1.00 USD mark, preferably slightly higher, if using the standard service.
Good luck with it.