Second, because online payments demand it. Third because this is by far the fastest. But most of all because nearly all problem customers are foreign or will try to dink around with money orders or checks. Use UPS where and when possible. Avoid using the post office.
Ideal sources are mil surplus skidded lots or distress auctions.
Exploit inefficient markets. If you hit on more than five percent of your auction items, you are paying too much. All commercial rights and all electronic media rights fully reserved. Reposting expressly forbidden. Instead, use product slots that we call threads here at my Synergetics.