Amazon requires a pure white background (RGB 255, 255, 255) for the main product image. The product must fill at least 85% of the frame. No watermarks, no logos, no text overlays. Listings that fail these rules can be suppressed from search.
eBay and Etsy are more relaxed but still recommend a clean background. Shopify and WooCommerce let you set a background color in the theme, which means a transparent PNG is often the best upload format.
The common thread is consistency. A catalog where every image has the same background color and the same framing looks professional. A catalog with mixed backgrounds looks like a flea market.