Start with How To Price For B2B (18 min, Y Combinator), then Your Prices are Killing You on eBay & Poshmark. The Magic $46.01 Rule. (10 min, Daily Refinement). All 8 lessons here are free, from the teachers' own official uploads. You'll need thermal label printer, kraft mailer boxes (about $145 of kit). The book to keep beside you: Profit First by Mike Michalowicz.
What you'll need
Thermal label printer: Print shipping labels in a second.