How can I tell if my 2024 Macbook Pro is genuine?

If you are going to keep & use that device, then you should definitely perform a clean install of macOS by first erasing the disk. Even better would be to perform a DFU firmware Restore if you have access to another Mac running macOS 15.x Sequoia. I highly recommend doing this even if they properly prepared the device for sale since you can never be sure if they left a nasty surprise:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Also, by performing the clean install yourself (or the DFU Firmware Restore), you may discover whether that laptop is still being managed by a previous owner. Many times you will either find you cannot erase it, or you will discover a message during initial setup of the new OS that it is being managed. Unfortunately this forum is full of posts where people only discovered much later that their preowned Mac was still being managed by a previous owner (there is nothing you can do about it).

Another issue buying a "new" device from unknown & unauthorized users/vendors is that they are considered the original purchaser of that device. If you accidentally lock the device, then you will be unable to have Apple remove the lock since you won't have the necessary acceptable proof of purchase to prove ownership. I have seen some reports that in some regions the original receipt is required for official Apple service (not sure if that is true, but I can see it as a possibility if stolen goods are a huge issue in a particular region). And the product warranty began when the device was first sold....which in this case you are not the first "owner" even if the box is factory sealed.

Without having the device in hand, there's no way for you to know if it's genuine. And even if it's genuine, it may well be stolen. This is a risk you take when you buy something sight-unseen from a non-authorized source. That the seller got high marks on eBay is no guarantee; scammers and crooks can all-to-easily find ways to submit fake reviews and ratings. It seems highly unlikely to me that someone would be selling a new model MacBook this soon, though it may well be legitimate.

In any case, you'll have to wait until you have the Mac before you can check it. Then you can check the serial number on the Mac against Apple's warranty coverage:

Check Your Service and Support Coverage - Apple Support

and at least see if the information that is shown at least seems to match the Mac you received. Of course examine the device carefully; a serial number on a sticker or that appears to have been altered would be a dead giveaway that something is amiss. Similarly, if it's not running macOS, then it's obviously a fake. Also, if it is a Mac but it doesn't come up to the normal Mac "welcome" screen, then it's probably stolen, or at least hasn't been properly prepared for resale.

Regards.

Yes, they can, or just taken from another device. That's why checking the serial number now, before you have the device in hand, will do no good.

It's not uncommon to see reports of stolen devices being sold on eBay, Amazon Marketplace, or other such sites. I haven't seen a lot of reports of faked MacBooks - that's more common with iPhones - but it's entirely possible. You'll just have to see what you get when it arrives.

Good luck.