This is where someone offers to trade their big item for an amount of smaller items that exceeds the maximum amount of items allowed in a single trade (20). For example, a scammer might want 60 Den Betas for their Black Long Collar. They will have their victim trade the extra 20 or 40 Den Betas for bad items. The victim will expect to trade for the Black Long Collar next, but the scammer blocks them.
This scam can also happen the other way around. A scammer can want to trade an amount of items that exceeds the trading maximum for a bigger item. For example, they may offer to trade their 60 Den Betas for someone else's Black Long Collar. If the scammer goes first, they trade 20 Den Betas for their victim's Black Long Collar (worth 60 Den Betas) and then block their victim.
How to avoid this scam
If you want to ensure that you won't get scammed, simply don't do big trades. If you want to do big trades, there's usually a safe method. For example, if you wanted 60 Den Betas for your Black Long, first have the other person trade you 3 Red Longs for the Black Long. Then they can trade you 20 Den Betas for each of your 3 Red Longs, making 60 Den Betas total.