This seems like an easy question and should be quite clear. I guarantee everything I sell for life. That is the life of the item. It has nothing to do with who owns the item as they were assuming the autograph is good.
Also, when you purchase an item for cash, that is the guarantee. If it is a trade, then that complicates it a little. It should still be a deal the owner of the original item is comfortable with or the value of the autograph when it was originally sold.
Lastly, the price paid for a 3rd party opinion should be the responsibility of the person paying for it. They want the opinion, not the seller. I sometimes sell items with those LOA's, but I have no responsibility to provide one. I only guarantee the item is authentic. Now if there was disagreement on the authenticity of the item and both buyer and seller were to agree on the opinion of the 3rd party, then maybe they split the fee.
Any other "guarantees" really aren't a guarantee or that's the way I see it. I sell many items to the end user and feel responsible to provide them an absolutely authentic item. Jason
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