Most marketplaces want products on a plain background, and a photo taken on your kitchen table is not that. Cutting the object out is the fix.
Separation is everything. A dark product on a light surface cuts out cleanly, and a white mug on a white tablecloth does not, no matter what tool you use. Two minutes spent finding a contrasting surface saves twenty minutes of tracing.
Edges are where cutouts fail. Hair, fur, mesh and anything transparent are genuinely difficult, and a rushed trace around them looks worse than the original photo. Slow down on the fiddly parts and take the straight edges quickly.
You trace it yourself here rather than handing it to an automatic tool. That is slower on an easy photo and much better on a hard one, because you are deciding where the edge is rather than arguing with something that guessed wrong.
No, you trace the edge yourself. On difficult photographs that is an advantage, because automatic cutouts tend to fail exactly where the item is most awkward.
JPG for a finished listing on white. PNG if you want to keep the transparent version to reuse on other backgrounds.
No.