Thread title includes second class relics, which are not body parts.
But first (and second) class relics have their character from what they ARE, not what has been done to them, and as such, even after they are sold for money, they remain what they are regardless of how they have been blessed.
Any blessed item loses its blessing the moment it is OFFERED for sale (before the sale and after the sail is the same thing -- blessing is lost), such as an agnus dei (made of bee's wax, blessed by the pope).
But this rule can be sidestepped as well by calling it a "suggested donation" instead of "price." BTW then you don't need to worry about sales tax, either.