A: Either you didn't confirm it in the mobile app, or your account has a pending restriction (e.g., new device, recent password change). Final Warning: The Steam Market is for buying/selling items , not for trading items with friends. For that, use the Trade Offer system (Inventory > Trade Offers). Never trade items for promises of PayPal money or external websites—that is how 99% of scams happen.
A: Instantly. The balance updates as soon as the item is purchased.
A: No. Only in-game items, trading cards, and Steam profile customization items.