Hey there! Just wanted to let you know that I merged your posts. If you need to, you can use the edit button at the bottom of each post to make updates. :) I also edited the link to the Ebay listing out of your post because it's technically against our rules.
But we can still help you out! Generally speaking, cards purchased on Ebay or Facebook Marketplace, or Craigslist, or [insert any place random people sell random things], you're always left to the whim of whoever posted the listing and whatever their intentions are. It could be that the image associated with the listing isn't representative of the actual product, or they're using misleading language, or misrepresenting the condition of the item. You're taking a calculated risk every time you use one of these sites because the possibility of scammers or people just trying to pawn off fake/cheap/used cards is ever present. In your case, it could be that the owner of the listing may just be unfamiliar with the appropriate pricing of a Charizard GX. It could also be that the card is in terrible condition. Always make sure you read pages and product descriptions thoroughly! Check if they have a positive history of sales and do some research into the seller.
On the other hand, using larger, more established sites that are in themselves libraries of Pokémon cards with their own internal stock can be more expensive because you're not as much at risk to some of the things I mentioned above. Websites like the one you posted seem to act as a middleman between seller and buyer are generally going to be more expensive because of a "finder's fee". So that website takes a cut of the profits and thus the total price goes up.
At the end of the day, it's up to you to make an educated choice on the matter. Oh, and a generally good rule of thumb to follow when buying Pokémon cards: if it looks too good to be true, it probably is. Good luck to ya!