I buy and resell things online for extra income. Trading cards are one kind of item I usually sell.
How you should sell them depends on exactly what it is you have. Do you have older cards or newer ones? How many rare cards do you have in your collection? If you have a lot of them (lets say a few hundred at least) then selling as a lot would be the best way to sell. If you have individual cards that are particularly valuable, then selling those cards on their own would be good, then sell the ones that aren't as valuable as a lot. You can probably get more money for them selling this way, but it will take more time and effort on your part to list and sell them. If you just want them gone in one go, then list them as a lot with the rare ones specifically mentioned in the title.
In terms of where you sell them; assuming you don't want to deal with people face to face (Craigslist) then I'd say eBay is probably the best bet. There are other eCommerce sites, but they are not as well known. Be sure that you read and understand all the rules on eBay before you list, and make sure you accurately list them. A scammers favorite target is a new, inexperienced seller who has no idea what they are doing.
I hope this was helpful. Feel free to message me with any other questions. I have been a successful e-seller since 2012 so I have a fair amount of knowledge I can share.