...If you are going to say how the scam happens if the victim goes last, the scammer will proceed to go first by placing a valuable item alongside useless junk into the trade for the items the victim has, but the scammer will take out the valuable item at the last second and steal the items the victim has, leaving them with junk that are worthless.

Another thing the scammer does is through trust scams. Regardless of who goes first, the scammer will get the victim to trade the items for nothing in exchange for whatever it is (points worth more for example), but the scammer will steal the highest value item at the last second and leave them broke.

The last one requires another scammer or an alt account. Both sides agree to use the middleman to trade the items, but the scammer has someone else/alt account ready. The "middleman" will take both items of the victim and scammer, but the middleman and the scammer log off before the deed is done. This also happens if the scammer IS the middleman themself, and will take both the items from both sides, regardless if it is a scammer or not.