Urban medicinal plant markets preserve traditional knowledge, but information on the quality and chemical characteristics of commercial plant materials remains limited. This study combined a descriptive cross-sectional survey of 48 medicinal plant vendors at Gamarra Market, Lima, Peru, with selected physicochemical, elemental, and phytochemical analyses of commercial leaves of Aristeguietia ballii (Oliv.) R.M.King & H.Rob., Borago officinalis L., Senecio canescens (Bonpl.) Cuatrec., and Eucalyptus globulus Labill. The survey documented plant names, reported medicinal uses, plant parts, preparation methods, and routes of administration. Laboratory analyses included proximate composition, arsenic (As), lead (Pb), and mercury (Hg) determination, preliminary phytochemical screening, spectrophotometric estimates of total phenolic content (TPC) and total flavonoid content (TFC), and DPPH and ABTS radical-scavenging assays. A. ballii had the highest concentrations of As (2.18 mg/kg) and Pb (2.54 mg/kg) among the materials analyzed, while Hg was below the detection limit in all samples. E. globulus yielded positive reactions in the greatest number of qualitative screening assays and the highest TPC in its hydroethanolic extract (53.22 ± 1.00 mg GAE/g DS) and TFC in its chloroform-derived extract (21.92 ± 0.41 mg QE/g DS). Hydroethanolic B. officinalis had the highest DPPH response (33.39 ± 0.63 mg TE/g DS), whereas aqueous E. globulus had the highest ABTS response (63.60 ± 1.19 mg TE/g DS). These findings characterize the interviewed vendors’ reports and the commercial samples analyzed. Convenience sampling and one composite commercial sample per species limit generalizability, and the chemical assays do not establish respiratory efficacy or overall toxicological safety.