I am a Jew and I sells on amazon, am I allowed to sell this item? http://www.amazon.com/dp/B002T2QUBA/
Yes, this is permitted.
Whether Christianity is considered idolatry or not depends on the exact denomination, and in any event the item in question (a pen) does not present any halachic problem vis-a-vis idolatry.
There is certainly no concern that such an item will cause a Jew to “leave the fold,” and therefore there is no problem in selling the item (which in any case can be obtained anywhere).