Here's how it works.
Lets suppose Valve lets you sell or gift games to each other in order to allow your right of resale. There's a flurry of activity as people want to get rid of their old games, either for cash or in free trade with friends. Gabe Newell is happy.
Until, that is, a hacker compromises his Steam account. He logs in one Monday to find that over the weekend, somebody donated away all his games to another account. He has staff close that account, only to find that several thousand dollars worth of games have already been sold to unwitting players who assumed they were buying legitimately.
Valve a hold on the account's funds transfer, but it's too late - most of the money is already gone from the Valve system and is on its way to a bank account in Nigeria.
Now, even if holds are placed to allow only one trade per game each month, there's still high reward in raiding inactive accounts.