I’ve needed to machine a lot of cast iron Chicago Dura Bar (very thick stock, like 10”x4”x2” bricks) for a product line requiring surfaces about .0003” flat. But only once I did order a good bunch of right-sized cast iron bar from eBay. Once. I had no idea the source and assumed it might be dura bar. As soon as I surface ground it, all sorts of pitting and inclusions came up ruining my product and its finish. I’d go deeper and deeper and more acne kept coming up. Un-sellable... I called the seller (in NC) who admitted it was from India (or was it Pakistan) and of course no he wouldn’t take it back - since principally - it was in fact cast iron. Just crappy cast iron. I’ve bought some correct needed size of various low-carbon steels and brass or bronze off eBay, but mostly rely on my local metals vendors where it has to be what’s advertised. And no, i wouldn’t buy a good carbon steel online unless from a known reputable seller.
And folks, no matter what, as soon as you get the steel inside, DO USE a Sharpie and mark it ALL OVER w the type/designation of steel or class of bronze, etc! You will forget in a year’s time what is what w all the moving about and re-shelving and cutting….and I do the same on the backs of my belts (for grit) as soon as I open the box, since many seem to have some weird designation. I forget all the time what is what and a Sharpie is a life saver.