I collect coins and I've noticed some people selling fakes (a fact which they don't hide,  they make it very clear they are not genuine). They list them as "fantasy" coins. Could you use a similar type word without mentioning bone or ivory? 

Or, you could list it as "the knife of your dreams!". There has to be some appropriate wording to describe it without the bots jumping on it. Another option could be to leave it be for a few months, even until the new year and then try listing it again. The bots may have moved onto someone else by then.

Don't worry about the trolls. Trolls live under a bridge and the bridge ends up collapsing eventually. Good luck!