Can Muslims buy things from any retailer no matter who the retailer is? Or how his pricing of particular products may be?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Transactions among people (sales, etc.) depend on mutual consent or agreement. As long as the consent is an act of the heart which could be declared by merely remaining silent then the Shariah considers anything indicating the approval of the transaction as an evidence for the acceptance of that deal.
Therefore, if a seller and buyer agree on the price fixed by the seller whether he is a wholesaler or a retailer the sale is valid and effective. The state of the contractor (seller or buyer), i.e. whether good or bad, trustworthy or cheater, etc. does not have effect the validity of the transaction. In fact, every transaction which is carried out by a sane person, i.e. not mad and able to discriminate between good and bad, is a valid transaction according to Islamic Shariah.
Allah knows best.