They have to wait 6 months or some similar long time after the last ebay activity, so almostr invariably ebay has booted them off for some reason which may vary from out and out fraud to late payment of accont to just bad luck or no known reason.
They can also be temporarily suspended for not paying their ebay fees -- had that happen with one seller I bought from -- it was a bit worrisome for a few days until the item I bought showed up. And I noticed a few days later that the seller was back in action.
