If you're new to the hobby and unfamiliar with items you're bidding on or planning to buy via mail order or such, NEVER, NEVER, NEVER pay attention to the "O", "O27", "scale" descriptions/designations until AFTER you have thoroughly researched the precise item you're thinking of buying.
I imagine 90% of the people selling this stuff--and especially on eBay and other such sites--have no idea of the meaning--past or present--of these designations.