Why these tools?
These tools are the easiest to use; you don’t require any experience. I have been using Remove. Bg has been doing excellently for me. However, you may need to upgrade if you want your images in HD format.
A paid background remover that has also proven to be very useful to me is Canva. Beautifully, they give you a whole month for a trial. Moreover, you can refer your friends to Canva and get access to premium features.
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If you want to offer background removal as your Fiverr freelance gig, I think you have enough information to get started. Just use one of the above online tools. They are 100% automatic and free for use.
Tip: Use the free versions of these background removers and get their premium versions as you get some orders. One Bg remover may not do a good job, but the other will do. Canva premium can be a good option for your Fiverr freelance gig if you want to upgrade.