Good question — here are reputable online places in the U.S. with strong selections of used / pre‑owned men’s watches, plus quick notes and buying tips so you shop safely.

Top marketplaces and dealers

• Chrono24 — giant global marketplace (500k+ watches) with escrow/payment protection and an authenticity/return policy for dealers and many listings. Good for wide selection and price comparison. (apps.apple.com)
• eBay (Watch category + Authenticity Guarantee) — huge selection (from affordable to high‑end). Look for listings covered by eBay’s Authenticity Guarantee (third‑party inspection on eligible watches). Great for bargains but verify seller history and returns. (eBay.com)
• Bob’s Watches — long‑running U.S. specialist (very large Rolex and luxury inventory), transparent pricing and in‑house authentication. Good for Rolex and mainstream luxury brands. (bobswatches.com)
• Crown & Caliber — U.S. dealer that refurbishes and authenticates watches, typically offers a (multi‑year) warranty and returns. Reliable for serviced pre‑owned pieces. (blog.crownandcaliber.com)
• WatchBox — large pre‑owned luxury dealer with global inventory and its own warranty/servicing; good for higher‑end pieces. (usa.watchpro.com)

Also worth checking (brand/price dependent)

• Hodinkee Shop (curated pre‑owned and vintage pieces) — strong curation for collectors. (help.Hodinkee.com)
• Watchfinder (UK-based, ships to U.S.) — wide selection of pre‑owned luxury watches, often inspected and warranted.
• Local reputable jewelers / independent watchmakers — sometimes best for hands‑on inspection, negotiation, and after‑sale service.

Quick buying checklist (most important)

• Authentication & warranty — buy from sellers who provide third‑party authentication or an in‑house guarantee/warranty (look for the length and what it covers). (pages.eBay.com)
• Return policy — prefer at least a short no‑questions return window so you can inspect the watch in person.
• Photos & condition report — request high‑resolution photos (dial, case back, movement if possible), service history, and whether the watch has original box/papers. (blog.crownandcaliber.com)
• Payment & escrow — use platforms with escrow or buyer protection (Chrono24 escrow, eBay protections) or pay by credit card for chargeback protection. (apps.apple.com)
• Serial/model verification — verify model/serial numbers against brand references and check for aftermarket parts or heavy refinishing if originality matters to you. (blog.crownandcaliber.com)
• Seller reputation — check feedback, Trustpilot/reviews, and shop history; avoid listings with spotty history or pressure to move off‑platform.
• Red flags — unusually low price for a hyped model, no returns, no provenance/photos, seller refuses authenticated shipping.

If you tell me the brands, models, budget range, or whether you want vintage vs. modern, I can search current listings and point to specific watches or sellers right now.