Aulus Gellius was a Latin grammarian and author who was most likely from Rome but was educated in Athens. He is most famous for is work Attic Nights. Here is an interesting reference to "used" books that he came across in a marketplace.
"When I was returning from Greece to Italy and had come to Brundisium, after disembarking I was strolling about in that famous port, which Quintus Ennius called praepes, or “propitious,” using an epithet that is somewhat far-fetched, but altogether apt. There I saw some bundles of books [libros] exposed for sale, and I at once eagerly hurried to them. Now, all those books [libri] were in Greek, filled with marvelous tales, things unheard of, incredible; but the writers were ancient and of no mean authority: Aristeas of Proconnesus, Isigonus of Nicaea, Ctesias and Onesicritus, Philostephanus and Hegesias. The volumes [volumina] themselves, however, were filthy from long neglect, in bad condition and unsightly. Nevertheless, I drew near and asked their price; then, attracted by their extraordinary and unexpected cheapness, I bought a large number of them for a small sum, and ran through all of them hastily in the course of the next two nights. As I read, I culled from them, and noted down, some things that were remarkable and for the most part unmentioned by our native writers; these I have inserted here and there in these notes, so that whoever shall read them may not be found to be wholly ignorant and ἀνήκοος, or “uninstructed,” when hearing tales of that kind." (Att. 9.4.1-6)
There a few insights that can be gleaned from this brief look at a bookseller in the Roman city of Brundisium (in the "heel" of the peninsula).
1) Because of the use of the word "volumina" it is most likely that he is referring to rolls and not codices.
2) The books were in poor condition and looked old "from long neglect," which likely contributed to Gellius's thinking that the authors where ancient.
3) Perhaps he knew some of the authors already, but most of them were "of no mean authority."
4) The books were inexpensive for he "bought a large number of them for a small sum."
5) Gellius valued the books because of the fascinating tales told within them, in other words, Gellius liked to read a good story.
This makes me wonder how likely it would have been for some Christian writings, for example, one of the four Gospels, or something like The Shepherd of Hermas to be available at a bookseller like this. This might explain how ancient pagans, outsiders, and converts came across these writings, such as Aristides and Celsus.
Gellius. Attic Nights, Volume II: Books 6-13. Translated by J. C. Rolfe. LCL 200. Cambridge: Harvard University Press, 1927.