Hi John,
Firstly welcome to the forums, it’s great to have you 
Now onto your questions. Bottom line it’s illegal to sell products with these characters without a license.
These characters will be protected by copy write and trademark.
THE STORES ONLINE AND EBAY
Many of these stores selling similar products would be operating without a license, and they are playing the numbers game, picture how many are out there selling around the world – up to a million on-line stores globally. (NOTE: Some may have a license – do not assume)
Those without a license, they are gambling on not being found due to the number of operating online stores and ebay – although ebay has cracked down on a lot of them.
The companies that own these brands – Disney (IE Marvel) Universal/Warner Brothers (IE DC characters, Hulk and Namor) Sony (IE Spider-Man)
20th Century Fox (IE X-Men)
All of these owners have entire departments dedicated to protecting their brand and be on the look out for illegal stores selling the likeness of their characters and when found the action is swift.
IF, you are wanting to sell DC or Marvel, I suggest your contact about a license, it takes up to 4 months, and you need to submit a proposal on what you plan to manufacture. To be honest the chances of success are very limited.
MAKING YOUR OWN PRODUCTS
Your Goal – My goal is to design my own products drawing “inspiration” of existing material.
Your example of “justsaiyan” is a good display of “heros” while not having recognizable similarity to any specific DC or Marvel characters, they are very generic – they actually did a good job.
So it is possible, but just be very careful and know all the characters, because:
Under copyright law, the standard for infringement is “substantial similarity.” Substantial similarity means an average observer would recognize that the second work takes copyrightable authorship from the first one.
This is just an example, so if you plan to design your own, I suggest you speak to a IP Lawyer with some knowledge in the area.
And as you are designing review every character – there are 100’s of them, and it can be overwhelming o_O
I know this post is long, but these brands are dear to my heart.
Good luck
Cheers
Mischelle
