A seller is someone who sells a product. Often the seller is following up on a lead produced by something called cold calling. Cold calling is when telemarketers or the sellers themselves conduct surveys about products, whether in person, by telephone calls, or online. That survey information identifies potential customers and sellers can then attempt to sell directly to potential customers.
A cold seller is someone who does not have access to cold calling information. These kinds of sellers fall under the larger umbrella of direct marketing. Direct sales or "cold" sales are typically door-to-door, or conducted through "parties" (candle parties, Tupperware parties, etc.) and sometimes through direct telephone sales or more rarely direct online sales. A traditional seller can focus directly on people they know are interested in the products being sold and have purchased them before or plan to buy them in the future. However, cold sales are far more challenging as most people you will encounter aren't really interested in the product.
Does that make sense? I hope so. Good luck!