Is a Jew allowed to sell kratzmach stuff on Amazon?
Yes, this is permitted.
Although the decorations are used on trees etc. the pagan origins of these customs have long been forgotten, and today they are merely part of the annual celebrations. There is no concern that they will be worshiped, so that it is permitted to sell them.
See Rema (Yoreh De’ah 141:1, based on the Mordechai, citing from Raavya) and Terumas Hadeshen (196) who write that even a cross that is made for jewelry does not constitute any prohibition of idolatry. Some are stringent concerning this matter (see Shach 6), requiring that we know the cross was not “worshiped.”
Of course, decorations were not worshiped, and it is permitted to sell them. Unlike the idea of selling animals to idolaters during their festivals, which we assume are for the purpose of sacrifices, with decorations there is no concrete concern for idolatry-related use, and therefore there is no problem.