Q: The question now is: Some of my customers are from the US, do I need to pay Tax on these sales ? even though I am not a citizen or resident in the US ?
A: Not if your income is obtained outside the US. For example, let's say you own a pub in Germany. If a US citizen visits that pub and buys dinner and drinks, do you have to pay US income taxes? Obviously not! So, the issue is "where is your income generated?" If you and your business are outside the US, and you are not a citizen, and the servers that host your website are located outside the US, then I submit that you wouldn't owe any US income taxes at all.
Q: If I start paying Taxes now for US Sales through the newly established LLC, (LLC Sole ownership is me). Will IRS question about the previous 2 or 3 years unpaid Taxes ?
A: The income would belong to the LLC. If it's newly established, then there wouldn't be 2 or 3 previous years. So the answer is likely no. And it's probably irrelevant anyway because of my answer above.
Q: Anyway I can find out if anything was reported ? Is there a way to know if I owe any Taxes without raising any flags ?
A: The only way to know for sure would be to contact the IRS. But in my experience, the fact that you used PayPal is irrelevant. The IRS does not review PayPal reports. Further, for the reasons stated above, I don't think you'd owe taxes.