Is it really dishonest though? The description makes it clear enough what you're buying - something dating from 1880 which contains shavings from something earlier, which comes with a statement from 1880 that the original was used in the cross. It doesn't claim any "powers" for the item, or that it is a complete original.
The description isn't actually misleading. What people feel about it is up to them - people might well want to bid on it simply because it is a Victorian religious relic with an interesting pedigree. If they want it because it makes them feel more spiritual, then that's their choice.