got scammed

[Morseton]

a dude was trading dbow for 2 items I had, when trade finished he kept my items and i didnt get dbow... wtf

[BananaNation]

He probably didn't even select the bow.

[BernardoLima]

Some people try to cheat on trade, they quickly check them then uncheck, so maybe you didn't saw that he had unchecked the D. Bow.

[lofi]

This usually happens when they ask you to modify the trade in some way.

Had a few scammers try to pull a fast one like this.

Always watch the trade window carefully if they ask you to add or subtract items from the trade.

[Madmayhem]

To elaborate, if they asked you to add, swap, or remove an item, then they probably deselected it at the very second you obeyed them.
All you can do is be more careful to avoid this check both trade windows whenever you change something, like lofi said