I was a university student on student visa (F1) in USA during 2001-2003 when I got social security number in USA and opened bank account there. Later in 2003, I droped from university and returned to my home country. I used this USA bank account for transactions (about half a million dollar in five years) of my eBay business through PayPal after returning from USA to my home country. I am again going to USA in January 2012 on student visa in another university. Do I need another social security number in USA? Will I be asked to pay or file tax forms when I go to USA for transactions that I did while I was not in USA? I did my business from my home country using USA bank. Does my new university know about my study records in my old universtiy? I did not disclose my scores of last USA university while applying to this new university. I am offered Teaching Assistantship by new university and I will get paid. What legal actions will be taken against me in USA if I fail to pay tax for money that I earned while I was in my home country? How much tax I need to pay?