A retailer is anyone who retails something - sells it on. The default meaning is usually associated with a shop of some sort selling in small quantities, but can be extended.
A wholesale retailer (which I cannot see differing from "wholesaler") is someone who only sells large quantities of goods. For example, he might sell you 500Kg of sugar, but certainly will not sell you 2Kg - that is the job of an ordinary retailer - i.e. a shop. To the wholesaler, 500Kg sugar might be a small amount.
Wholesalers obtain preferential rates from manufacturers/suppliers/importers and then ordinary retailers (business/shop owners) will buy the goods and sell them on to the public piecemeal.
A wholesaler (wholesale retailer - someone who only 'retails' in large quantities) may also sell to those who use the goods in large quantities but this is usually the province of a distributor, manufacturer, supplier or importer.