Amazon, eBay and most marketplaces require a pure white (#FFFFFF) background on the main product image. Editing the original photo rarely gets you there; cutting the product out does.
Cut first, then fill
Remove the background, then apply a solid white background in the Background tab. The result is mathematically pure white rather than 'nearly white'.
Check the edges
Dark halos are the giveaway of a rushed cutout. Nudge the halo reduction and edge cleanup sliders until the fringe disappears.
Export
With a solid background applied, JPEG is fine and keeps the file small.
Full instructions are in the background guide