A scammer will tell you their item of lesser rarity is the same item as a similar, rarer item, and ask for the rare item. An item commonly used to fool people is Brown Tile, which is identical in appearance to the Tan Carpet. However, Brown Tile has the member lock and Tan Carpet is nonmember. Since Tan Carpet is Beta, it's worth 3-4 Den Betas, while Brown Tile is storebought.
Someone may also say that their item is unreleased or glitched. For example, there is an unreleased Snowflake Balloon that is blue and pure white. It is worth a Black Long Collar. However, there is a similar blue version that is only worth RIMs. Scammers may lie about the worth of magenta items (ones that aren't very rare), or claim that items are Beta when they're not.
How to avoid this scam
Before doing a trade, be sure of your information. Use the Animal Jam Item Worth Wiki and the Animal Jam Wiki to compare different items, see what items are worth, and find out which items are actually Beta.