For a similar type of scam, see Gift and Trade Scam.
A scammer will offer to trade up one of your rare items, claiming that they want to help you get rarer. However, when you trade them your item, they block you and never return it.
How to avoid this scam
Don't take the risk. Someone may act like they're trying to help you by making this offer, but why don't they just give you rare items if they want to help? They just want to take your rares.