Some peoolle have automated ebay/amazon leveraging set ups - for eg the cheapest I could find a brand name snorkel set on ebay was £19.99 - and since i often find ebay the cheapest, I bought without futher searching.

3 days later the package arrived from amazon, complete with packing slip, where I found it cost £16.

Searching the sellers account they had thousands of random listings - where I assume they can leverage a small profit. Items came and went quickly from their inventory, I assume as amazon prices fluctated.