Hi drabagchi,
if a seller is selling a product significantly lower that its market price, then i would be hisitant to buy from the sellers. A deal that looks too good to be true, probably isnt true.
In such a case, I would look at seller feedback ratings and comments. If the seller has lot of positive feedbacks from selling similar products (mobile phones in this case), at similar price points, only then I will trust the seller. It is also advisable to ask seller question and calrify all the questions that you have in mind before buying.
I hope the above helps!
Regards,
Sapana