Library sales are ideal for lotting because donations often arrive as whole collections: a full mystery series, a shelf of a single author, a run of a professional journal, or an entire homeschool curriculum. Sorting volunteers rarely keep those together, so part of the skill is recognizing and reassembling sets.
Complete matching sets sell for far more than the sum of their parts, and matching means more than title continuity — same imprint, same cover style, and consistent condition all raise the price a buyer will pay.
Lots also solve the mass market paperback problem. Twenty individual 50-cent paperbacks are unsellable one at a time, but the complete series in one box is a listing worth making, especially on eBay where multi-item shipping is straightforward.