Although the original Flash scam was prevented when Animal Jam changed trading, a new version of Flash is still possible, and is commonly used to scam.
In Animal Jam Play Wild, it is no longer possible to do this method as the new trading system has changed, nullifying this scam in the process.
Flash (Also known as Double Acceptance or Accept, Accept, Decline) scams are often hosted as a giveaway. You're told to do the first accepting step of a trade, then the next step, but then decline really fast. (Accept, accept, decline.) A scammer will sometimes imply that there's a 3rd confirmation step (sometimes even saying that Flash Trading has no risk), but a 3rd step doesn't exist. After you accept twice, the scammer can easily complete the trade and take your item.
How to avoid this scam
Never participate in Flash Trades! Although flash was originally a party game, it simply isn't anymore. Flash is a method of scamming, so don't participate in it, even if it's with a buddy. If someone hosts a flash giveaway and it seems legitimate, it's because they're using all the right tricks (returning items, spare accounts, helpers, etc).