A scammer on YouTube say that they're giving away a large item, and that you have to give them rare items in order to enter. However, they never give the prize away. They give it to a spare or buddy, ignore the giveaway entirely, or abandon their channel.
A YouTuber hosts a "giveaway" which is actually a scam. They use common scamming tactics, or ask for larger rare items in order for you to be entered in a big giveaway which they never host (or they give prizes to spares and helpers).
A YouTuber gains trust, a good following, and a good reputation. People trust them enough to take risks with them, but they eventually get scammed. A YouTuber with a good reputation also may host rigged giveaways or scam giveaways, asking for rare items to be entered. Any large prizes are given to spares and helpers.
How to avoid these scams
If a YouTuber asks for rares, don't give them any you mind risking to a scam. If bigger, more well-known YouTubers start asking for large rare items, it's likely that they're scamming, using their solid fanbase and good reputation to their advantage. Even if someone seems legitimate, a scammer is a scammer. If someone is really giving away something, they shouldn't ask for your rare items.
Note: Some YouTubers are honest and receive small items for giveaways without scamming. It's reasonable to ask for a small entry for a large giveaway. You have to use common sense and discernment to decide who is trustworthy.