Oh, my apologies. I don’t see spending money on ads as a viable option for your store to be totally honest. Maybe in a marketplace setting with low CPCs and very specific search queries, but in Google Ads it will be very difficult.
I would rule out Facebook Ads entirely as it really needs a consistent flow of ad creative and a definite number of conversions to properly optimize.
If you went Google Ads you would want to target only the people searching for your exact products, e.g. “Refurbished/used x phone” and you’d have to close that ad out when the item sold.
All that to say, in GAds, your budget is totally influenced by the amount of search volume available for those very specific search queries. I can’t really say without carefully studying your product catalogue and the keywords associated with them and their volume.
But it sounds to me like you’d want to start with a small budget, the great thing about Google Ads is that you can budget anything you want to it, as low as $1/day. So your budget is entirely in your control.
How much “should” a small business spend is not a 1-size-fits-all kind of answer. My agency has worked with small businesses with fewer than 25 items in their catalogue and spent 10s of thousands monthly, so it’s really a individual matter.
Sorry for the not-so-helpful answer. But I hope my reasoning for the answer helps. 