It sucks but I'm sure this has to do with preventing fraud. It would be suspicious if some inactive account just started buying hundreds/thousands of dollars of items that hold no 'real' value. Then you wind up with a charge back from a stolen credit card and a seller without their items.
This is why I actually give steam support props. They allowed me to access my kids account simply by telling them my other account name and that we probably shared an IP at some point as proof.
I realize it's probably a security hole but I appreciated that they got my kids account unlocked for me.
I ran into this after not buying anything new in a while (really don't need new games) If you have anything in your steam wallet from past items being sold, you can use that for the requirement of buying something, look for just a cheap classic, usually they can be $0.99 or so on steam sales. I found that works to get back to being able to trade again
