I've just read about this "SAB" issue, no direct experience. Regardless, there's a risk that needs mitigation.
How much can you trust the seller? Does he/she have decent selling history on a forum or long-term history on eBay? If there's not a compelling reason to trust the seller, steer clear of the deal with this series number.
If you can trust the seller, be sure that a sales receipt will be included. Fuji is covering parts but not labor, if you have the problem, and have the receipt. Find out what that parts cost is likely to be.
I'm not sure what percentage of the 12-series X100 units develop SAB, nor do I know if the likelihood increases, decreases, or stays the same that a particular X100 will develop SAB over time. Try to find this out. Perhaps it's a situation where if it hasn't had the problem by now, it probably won't - that's of course your lowest risk scenario. The worst scenario would probably be any particular 12-series being more likely to get the problem the longer it's used.
Bottom line - you're asking yourself to accept some risk for the potential benefit of lower cost. You have to know what the likely risk is, and then know your personal tolerance of that risk.
Best of luck should you decide to go for it! Do a bit of "Googling" homework first.