There are two questions that require answers:
Is the item on sale genuine in the sense that it was originally the property of a close confidante of King Louis XV of France? I have registered on eBay and sent a message to the seller requesting further information on the history of the item.
If the item was indeed the property of a member of the French court, I believe we can assume that the owner believed that the relics in it, including the purported Shroud relic, were genuine. We should therefore keep an open mind and accept that there is a possibility that his belief was correct. If, as the seller suggests, the item may have been a gift from the King, then the likelihood of the relics being genuine becomes much higher.
The purported Shroud relic is described as "Sainte Suaire". While there were indeed a number of shroud copies in France, I would think that only one item would be definitively described as "Holy Shroud". Other researchers may wish to comment on that.
With regard to price, I have no doubt the seller has set the price on the basis of what he believes the historical and artistic value of the item to be. The relics themselves would have little or no monetary value. While we as Shroud researchers might consider a piece of the Shroud to have great value, I would suggest that a dealer in 18th century art might take a very different and less exalted view.