I've never had to use the promote option, but I can see how it would be beneficial.
In particular the 3+ day promote.
It is an investment, which I would personally only recommend on $$$ items.
It will get more eyes on your item for sure, so do some math and see if it's worth it to you.
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Although, note that you are able to repost weekly or bi-weekly without having to pay anything.
The reason to do it this far apart is to not get flagged by a spam bot.
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If the want / need is there I'm sure you will eventually sell.
What are you selling by the way?
Some item simply just sell faster than others regardless of how long they are on the front feed.
Keep in mind once an item is gone from the front feed, potential buyers will have to search for them by name so;
Make sure your descriptions and titles are short and accurate + images clear enough to decipher.
