If he hacked your account, as in having complete control over it, why wouldn't he take everything in your backpack?
What it sounds like really happened was that you tried to sell your unusual hats for money and you traded them first expecting him to pay you afterwards.
The rest of his things on his Custardo account (if he is custardo, if not, he's GoldenWrenchie) are worth 90 ref, which is only around 7 keys, considering Backpack.TF's price. However, he stole more than 3 buds worth of stuff, so why would he take everything else when he could take more?
However, I am slightly suspicious on how the GoldenWrenchie account is F2P...
And I did the math, the unusuals that are worth in keys are 36.59, which is 2 buds. The rest in the trade he has,
located here, are worth 3.6+ buds (One is missing from the backpack, but the Steam API is down so), so that makes it >5.6 buds<. That's a lot more than 90 ref.