Two buttons, one question every seller eventually asks: does it matter which one I press?
It does. One of them gives an old listing a fresh start in search. The other one mostly does not.
What Relist does
Relist takes an ended listing and puts it back up under the same item ID. Same listing, same history, same view count, same age as far as eBay is concerned. It is fast, it keeps your watchers, and it is the right button if a listing ended by accident or ran out of duration and you just want it back.
What it does not do is make the listing look new.
What Sell Similar does
Sell Similar copies the listing into a brand new one with a new item ID. Title, photos, item specifics, price, everything carries across, but eBay treats it as a fresh listing. It gets the new-listing boost in search, it starts its age from zero, and it appears in "newly listed" sorts again.
You lose watchers and the old view count. On a listing that has sat for 90 days, that is not a loss. The watchers were not buying.
Why freshness matters
eBay's search leans toward recent listings. Two identical items, one listed yesterday and one listed 100 days ago, do not rank the same. The old one has also told eBay something: it has been seen and not bought. Every day that continues, the signal gets worse.
Sell Similar wipes that. It is the closest thing to a reset button eBay gives you, and it is why experienced sellers cycle old stock through it on a schedule.
When Sell Similar is the wrong answer
Here is the part most guides skip. Relisting a listing that is priced wrong, or that nobody wants at any price, does not fix it. It buys another 30 days of the same nothing, plus another round of fees if it is promoted.
Before you Sell Similar, ask two questions:
Is the price the problem? If comparable solds are $30 and you are at $45 hoping, a fresh listing at $45 is still a $45 listing. Drop the price at the same time as the relist, or do not bother.
Is the item the problem? Some stock does not sell on eBay at any price in any month. A relist cycle on those is a slow way to find out what a sold-comps check would have told you in a minute. Write it off, bundle it, or take it to the market.
Sell Similar works on stock that is fine and got buried. It does not work on stock that is wrong.