1) Is it permissible to buy raw chicken / meat from a retailer who also sell pork. Retailer has halal certificate.

2) Also what are the rulings to buy halal cooked food from retailer who also cook/sell pork?

1- If you are confident that he is selling halal product, then you can.
2- If he is not using the sme cutlery for the pork, then you can.