If you know that the mobile was purchased by seller not stolen then it was halal for you to buy the said mobile. As far as the matter of cheap price is concerned, the seller has the right to sell on low price:
لأن الثمن حق العاقد ، فلا ينبغي له أن يتعرض لحقه . (مجمع الأنهر 4/215 ، كتاب الكراهية، ط: غفاريه کوئٹہ پاکستان بحواله فتاوى محموديه 16/164 ، ڈابھیل)
However, as for the doubt that the seller might have not paid the import duty, this will be the responsibility of the seller.
Note: If the seller has not paid the import duty it shall not put any impact on your purchase and it shall remain halal.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband