Here are some random thoughts:
For me, eBay is where I do all of my sales. The fees are higher on Amazon and the competition is fierce. All sellers on one page works well for buying on Amazon, buyers being able to compare prices so easily means that if you aren't the cheapest, you aren't selling. You are not eligible to be in the "Buy box" unless you get a pro subscription at $40 a month, and even then you need to maintain good feedback and prices to get the buy box.
On eBay there are tons of people selling the same thing but you've gotta hunt for the lowest price. Being cheap is still the #1 priority but having a listing that stands out also helps. On eBay the more of the same item you sell the higher your ranking. If you've sold 30 of something at $9.99 and someone comes in cheaper you'll still be the first hit.
If you can order it wholesale so can everyone else. A good profit margin for the stuff I sell is 15%.
Be prepared to get scammed. Ebay and Amazon give the buyers all of the power so if someone says they didn't get something they'll get a refund, even with tracking confirmation. You can fight it but even if you win the buyer will likely leave negative feedback.
Ebay gives you a 20% discount on final value fees if you're a top rated seller, meaning you maintain a certain sales number and better than 98% feedback. Not letting a listing sell out is absolutely vital to keeping your ranking. If it sells through you have to start over.
On amazon automatic repricing software is an absolute necessity, on ebay not so much. Just be sure to check your competitions pricing on the regular and make sure you're not being undercut.
Dazzle is a brilliant shipping program that imports all of your addresses and weights (for Ebay). The $15 a month subscription gets you access to commercial base pricing via USPS, which pays for itself in no time. It also uploads tracking numbers for you. There's a free version that uses retail pricing for folks who don't ship much.
Like regular retail good customer service pays off. Kill them with kindness if there's a problem.
People like to complain about insertion fees, they are really a non issue.
A "Basic" ebay store gets you the same selling fee discount as all of the others. If you're listing scads of stuff is the only time getting a more upgraded store is worth it as they have more free listings and lower listing fees once you run out of freebies.
Some distributors will only sell to you if you have a retail front.
Shipping next business day at the latest is 100% a necessity.
USPS gives free priority supplies, which is nice. Keep in mind that paying for shipping supplies eats profit FAST.
Look up Ebay's Global Shipping Program. Use it. Love it. Not having to deal with international sales directly is awesome. The program is also free for sellers.
Dead product sucks, don't get in over your head.
This label printer is the E36 M3:
http://www.amazon.com/Zebra-GK420d-Monochrome-Desktop-Resolution/dp/B00EUN90SG/ref=sr_1_1?ie=UTF8&qid=1433887889&sr=8-1&keywords=zebra+gk420d