Why do many vendors only accept crypto or wire — is that a red flag?

It's a yellow flag, not automatic proof of a scam. Many peptide vendors are denied normal card processing because banks classify the category as high-risk, so they fall back to crypto, wire, or e-checks — a real, non-fraud reason it's common. But it matters for your protection: credit-card payments give you chargeback/dispute rights if you're billed wrong, never receive the item, or get a defective one, while cryptocurrency and wire transfers are effectively irreversible once sent — the FTC notes scammers prefer them precisely because the money can't be recovered. So crypto-only isn't damning by itself, but it removes your safety net; weigh it against the vendor's testing transparency and reputation, and know you're accepting more risk.