Respectfully, these are massive red flags. I wouldn't get involved with someone advertising a private chat group on eBay, and if you can't find the post it's possible it was deleted by someone who shares my suspicions. If you join such a group and you don't see them scamming someone, they're probably scamming you.
I get WhatsApp messages from time to time inviting me to join crypto groups with "investing tips" and the like — delete them immediately, every time.