Sorry if I came out this way, but this is my honest answer - the data is cheap... being a lazy person, for me everything is a risk/reward... $100 to back test any trading strategy I can imagine up to 20 years back is the best risk / reward, much much, better than to go and start trading "for free" on a fancy app on a $500 phone with a "small account" I can "afford" to loose...

I guess the reason why it seems confusing is because if you're prioritizing risk/reward tradeoffs and saying that spending the money for good data is worth it to you, why not license the data from a reputable source?

That said, I think I might not have conveyed my original question well enough - what I actually meant was 1) is the data accurate? (you sort of answered this by saying you compare it to presumably licensed sources) and 2) what actually led you to searching for financial data on ebay? (you didn't answer this, other than "it's cheap").

I'm basically wondering how you did the mental calculus that concluded ebay was a good place to check before you verified the data was alright.

dsacco, the reason why eBay of all places is simple, I like to buy cheap stuff on eBay, especially books, so I am always searching for books on the investing subject, so I can get ideas to back test and this is how I first stumble on the data and since the price was so cheap I got it and I tested it to make sure it was OK.

If it was not, I would have notify the seller, eBay and PayPal and get my money back...

Before this data, I was using Quntopian, but the huge limitation there is that you cannot test on the whole market, you cannot download their data, etc...

Anyway, I am trying to give an honest advice here and to prevent people of losing money because they don't know what they are doing...