Depends what you're selling.
Amazon generally needs a UPC or the product has to already be in their catalog. It's good for new, boxed items. Used items you got to be careful and pay attention to what the Amazon description says exactly. I sold a printer once on there, clearly said the print head wasn't included (and I was selling it for like 100 bucks when with heads it was close to 700), guy bought it and then complained it wasn't as described. Amazon let him keep the item and refunded him because they read the ad and I violated Amazon policy by stating that there were no print heads included. Amazon holds all funds for 21 days and as far as I'm aware will make no exceptions.
eBay you can sell most items you want, it's generally better for used items. Some things you can't sell like cop car computers (the regular PC from the car) as they claim it could be used to impersonate law enforcement. Not sure how you'd do that, but it's what they claimed. eBay holds money until 3 days after delivery or until your account is decently old with a selling history and you'll get paid instantly.
I would use both because at some point you'll end up pissing off either one of them and shouldn't have all your profits tied into one business.