I was wondering too. Regardless...
As far as tracking your wants and cards for sale, this site is second to none in my opinion. I think many others here will echo that. The different lists, the reports you can run, the functionality of it (once you get used to it) - yeah, this is the place for tracking your collection.
As far as actually selling, Pinkpony1967 is correct with his assessment of the other platforms. More members here would rather trade than buy, based on my observations and experiences. That's not to say that you won't be able to sell here - as long as you've got something to move for a reasonable price, it'll go. But one thing you'll notice is that many here have a ton of cards they want to get rid of, and they'd rather part with their extras and traders, even to a large ratio, than with money.
Outside platforms all have positives and negatives, as previously mentioned. Check out fees, and understand how the different sites are set up. There's likely lots of members here who can help with that; just ask.
What many do, and I've started myself, is to sell on eBay but link back to here. NJDevils is a frequent forum poster who sells on eBay. He will link his eBay auctions in a thread, and I am one of many who have purchased from him here. There are plenty of other users who also do that. For me, my COMC collection lists my "For Sale/Trade" list that is only available on COMC. I use the "External Sale" link to link to them and that allows site members to go right to my offerings.
Short answer: Collection tracking? Best site in the world. Actually selling your cards? Use TCDB as much as you can, but you'll likely experience better success with other outlets.
I'll never quit collecting entirely, but I am downsizing. Check out my COMC store and help me thin out what I don't want so I can buy cards that I do want. See something you like? Send me a message on here, and we can knock the price down quite a bit. I'll even take a bit of a loss if it means getting you a card you really want.