I don't have an ebay account as I was rejected, so I only use amazon. I started doing well with amazon once I started writing product hubs - I'm currently at about $600 for this month so far.
My advice would be to pick 3 niches you are interested in and write 10 hubs within each. e.g. overall niche of kitchen appliances = write 10 hubs with titles such as best food processors, best smoothie makers, top 5 cheap microwaves, retro toasters, best front loading washing machines, etc. (This is just an example, you'd have to do some research on what niches have the most potential).
Over a period of time you'll find out which niches convert best, then you can write more within that area.
When writing for amazon sales always choose products with high customer ratings. I usually select 5-10 products per hub and lay the hub out in this format: 200 word intro. 100 - 150 words per product each in a new text capsule with a photo on the right and the amazon link underneath, and a 150 word conclusion.
You'll need a good understanding of onpage optimisation and implement it on each hub.