Subset of a previously noted scam, in how the checks (in this case, cashier’s check) end up working against you, specific to a commonly used forum, Craigslist. Linking again to an awesome explanation given early in the thread.
All of these are sent by bots. There’s no one individually responding to posts (which is why they use generic words like “the item” when responding). They don’t want the furniture and no one will ever show up at your house because the true purpose is extract real money from a fake check. After you cash the (fake) check they will either ask you to send the “extra” money to their “courier” (actually the scammer using a different contact) or ask you to buy gift cards because they are buying your item as a present for someone else and want to include the gift cards with the “gift”, then later ask you to scratch off the gift card and send the code(s) to the scammer. They may try to claim the transaction is being “handled” by eBay, Amazon, or another well-known brand, which is completely false. These companies only provide transaction support for products sold directly through them.
Even if you do cash the check, which you should not, your bank will appear to credit the full amount to your account. This happens because the fake check is in the form of a cashier’s check, which banks are obligated by law to credit immediately. But the fraudulent nature of the check will eventually be discovered over a few days or even weeks, at which point your bank will remove the entire amount of the check from your account, possibly causing an overdraft to boot.
All Credit to @Cleophus - just wanted his entire message here for the searchability. From thread Is everyone on craigslist a scam artist?