Amazon's used conditions are more specific than eBay's, it also has special collectable conditions which you must get approval for (here).
The error you often get is that selecting a special, specific condition is not recognized by eBay - but picking the generic, used condition shows up on Amazon as "Used - Acceptable."
Create a Custom User Field and map it to ConditionType in Amazon XML Mappings. For help mapping Custom Fields to Amazon's XML Feed check out the SureDone Guide on How To: Use Amazon XML Mapping.
The acceptable values for this field (typed identically to the way they appear) are:
[New, UsedLikeNew, UsedVeryGood, UsedGood, UsedAcceptable, CollectibleLikeNew, CollectibleVeryGood, CollectibleGood, CollectibleAcceptable, Refurbished, Club]
