Guaranteed Delivery does not appear to affect eBay search ranking. There’s certainly no statement from eBay on that, and no other evidence that it has any influence on search.
Instead, it’s all down to the filter. If you have opted in to Guaranteed Delivery, and your listings are eligible, you will have a definite advantage over competitors who are not in the program. Once the filter is turned on, only eligible listings are shown. Other listings will be completely hidden from buyers.
If the filter does catch on with buyers, that should bring you more sales. That could also increase your sell-through rate and your ranking for all searches – filtered or not. So, while it may not have a direct impact, it could give you an indirect boost.
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