Logic....lets leave logic out. :)
The tensile strength listed by the mfg's concerns the sideplates only. There could be pot metal in the other components for all we know...although we are fairly sure that 20 mules and a driver couldn't break the sideplates.
Yes, all mechanical properties of metals relate to one another, but in the instance of choosing roller chain material, I would be concerned with the hardness of the bushing and pin material. Even further, I'd be more concerned with the compression yield factor. Which is kinda, sorta, the opposite of tensile strength.
Tensile stength, btw, is the measure of what force it takes to bring a metal from elastic to plastic (or breakage) state.
The compression yield is important because there is very little surface area that the chain pin actually comes in contact with the bushing, due to the variance between pin ID and bushing OD. This small area must be tough enough to withstand shock load as well as abrasion.
Using a lubricant with an EP(extreme pressure) additive will help the chain withstand both shock load and wear much better than a conventional film type lube. But once the barrier of any sort of lube has been broken, the mechanical properties of the metals are all that you have left. The better the numbers on the pins and bushings are, the less wear you will get in X amount of time as compared to a chain of lesser materials.
IF you can manage to use a lubricant and method that keeps the chains friction area's protected, you can get good service life out of "cheap" chains.
Now, back to the point....
When I talk cheap chain, I am usually reffering to the lesser-cost chains of a given brand...like the standard DID chain as opposed to the higher-dollar super-hard top shelfer DID's.
When we talk Ebay chain, you could have any sort of crap that got turned out in Bumphuzel, Malasia or similar. No way would I trust it to be of performance quality. I would be very leary of the quality control of this chain.
Sloppy tolerances throws everything you know about good chain care out the window. It's a hopeless battle to get good life out of such chain.
But, if you like riding instead of wrenching, and you wouldn't be upset if someone called your bike a "beater", then ebay chain may work out fine.
ALSO....some on you may want to be very cautious when purchasing chain on Ebay... You may want to do a little math before you hit the pay button.
I see where one putz is selling chain and estimating it's length way off. Plus, even his estimated length would leave you coming up short, with a orphan length of chain left over (two full chains and several key chains or Christmas ornaments).
Not much of a bargain on the back end.