Stump fires can last a long long time, and run thorugh the roots. I set a fire here last march on a field, and it caught an underground root and ran back to an old doug fir stump. Next time I was up there, there was a 20 ft blaze going as the stump was burning. I had to fill the 500 gallon truck tank with water and go up and put the fire down... learned later from some locals that stump and roots can smolder for months underground. So I do not burn stumps in the spring here any more, to ignite things here in summer. I only set them in the fall, as we have a long and wet rainey season here (43" so far since September this year).
As for stainless OWBs, several factors are bad about them. Our dealer talked us out of it, even though he could make $1000 more selling them. Stainless does not conduct heat as well as carbon steel. Stainless also has to be welded just right, and cool (shrink) without cracking. It tends to be brittle. It is far harder to fix if it does crack. Also if you take care of your boiler water with anti-corrosion and replace it every 3 years, it will not rust out from the water side. On the firebox side, water pockets can be a problem if you do not scrape down the side when creosote builds up, or scrape the ash pan regularly. I researched creosote some years ago and found that the corrosion factor for that on steel is very low and mild. Ashes themselves are also non-corrosive. Either mixed with water and you may get problems. During the off-season can also be a problem if you do not cap your stack and you get rain water in there.
As for buying OWBs on Ebay, I would completely avoid that. Never buy from a zero sales Ebay lister, unless it is a $5 item. They have no reputation to uphold, they have no feedback to establish integrity. Many times they are really someone with bad Ebay feedback that just dropped off of Ebay and re-signed with a new listing name. There are many many many scam artists out there, and some real junk OWBs sold on Ebay that are fly-by-night designs. Same with chainsaws... or anything mechanical that takes maintenance. Like with chaisaws, you are probably better off buying an OWB that is new.