At a sale, you can tell them apart across the room. Trade paperbacks stand as tall as most hardcovers and hold their shape; mass market paperbacks are short, thick, and tend to curl at the corners with cracked spine creases.
Condition behaves differently between the two. Mass market paper yellows and goes brittle within a decade, so spine cracks, tanning, and cover curl are the things to inspect. Trade paperbacks hold up far better but scuff along the flat cover surfaces and bump at the corners.
Pricing impact is stark. Mass market paperbacks are usually the cheapest bin at a sale — often 25 to 50 cents — and rarely worth reselling individually because shipping cost exceeds resale value. Trade paperbacks, especially current textbooks, test prep, and niche nonfiction, carry real value.