Steam Marketplace: 200 items in a calendar year ???

Unless you've attempted to pay with a us address in the past. (ie: prepaid usd credit card)

Then if you answer no, they'll want a bunch of documentation from you to prove you're not an american.

doubt that's true

Yes and no. Steam is required by law to collect the info once you hit the 200 sale mark. If you sell 200 items and made $20,000 from all those transactions, then yes, you are required to pay taxes. Valve is not required to share this info with the IRS though.

This is coming from the community marketplace's FAQs

Is there a limit on the number of transactions I can participate in in the Community Market?

Currently there is no limit on the number of transactions you can participate in. However, the United States Internal Revenue Service, under Section 6050W of the Internal Revenue Code, requires us to collect certain information from you if you engage in more than 200 separate sales transactions in a calendar year. In particular, if you are a U.S. citizen or resident, you will need to provide us with your name, address and Social Security Number, and if you are not a U.S. citizen or resident, you will need to provide evidence of your non-U.S. status on an online form we provide to you. We will be providing a link to these forms and a process by which you can provide the needed information if and as you approach the 200 sales limit. We may use a third-party to collect this information. You can choose not to provide this information if you wish, but you will not be able to list any items for sale in the Community Market once you meet the 200 sales threshold, and any items listed will be removed. Note that, while we are required to collect this information from you, we are not required to share this information with the IRS under Section 6050W unless you exceed $20,000 in gross sales in the Community Market within a single calendar year. For purposes of calculating the number of sales you have made in a calendar year, all of your accounts will be treated as a single account.

they ask everyone, you can put nonsense to your steam profile

even if you are older then 18 years they will ask every fcukin' time for your age for some games