I've been doing some research on slate/shell stones over the past few weeks, and have discovered some off-spec listings on Ebay with significant discount. I'm not totally sure how something can be labeled as brand new, snow grade but then discounted as off-spec at less than half the normal retail price. What kind of imperfections can I expect with these products?

My first thought: don't do it.
Without knowing what kind of flaws and how many, you will regret purchase.

It can have many kinds of flaws.
- limited number of stones
- chipped stones, damaged stones
- mixed stones (snow, moon, standard)
- irregular size of stones (diameter, or diameter not correct)
- irregular shaped stones (too flat, too bold, not proper convex, sharp edges, etc.)
- discoloured stones
- etc.

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Maybe get in contact with or look in threads by user Erythen? He often offers “refurbished” Go gear, much of it of good quality (as far as I can tell), and he has recently opened a shop on eBay, IIRC.

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Thanks for the replies. Erythen was kind enough to send me some images of what to expect. The flaws do seem significant on some of the examples he showed. I will probably pass on them given they are still rather pricey.