Study what is the most popular iPhone models in your country. You can investigate your country’s marketplace and see what iPhone models most sellers are selling and what grading is the best selling. Don’t just looking for price, because a good deal turn out may not be a good quality source.
Make sure the wholesalers are legitimate, experienced. Try to place a small sample lot to check their grading standard and quality. Talk to the salesman and see if they understand well the market, ask them question on how they source their phones and how they test the phones.
Check their warranty period. Make sure they provide warranty and support instead of walking away if you received problem phones. Many USA wholesalers you met in exhibitions may be doing business for years, but they never provide any guarantee once they ship the lot out.