Niches to avoid if you have an online store?

I need help in a certain area of choosing a niche. It's not just to do with choosing a niche per se, but, specifically one for your e-commerce.
I was wondering if there are niches that sell very very well on ebay but don't sell good on private websites? What i am saying is, for example, lets take a weightlifter who is looking for wrist straps in the gym, he may not have an account on ebay or amazon but instead types what he is looking for into google and clicks on the results, if he finds something, grand, uses the credit card and buys it. Now, if a mother at home decides her young child could do with some new lego sets, i can just imagine her logging into her ebay (or other) and buying through that instead of google search engine to find any old website selling it.

For me personally, my uncle who isn't "a computer genius" would generally buy from websites by typing what he needs into google, my aunt who is a little bit more smooth tends to compare and contrast a bit more before she buys and users ebay and amazon, so giving the fact that they also tend to buy very different products, i understand there has to be some niche products that are more suited to ebay as their customers are generally ebay users and some niche products that are more suited to e-commerce as their customers and generally "google search engine" users.

Please share your opinions and feel free to give some advice on which niches may be more suited to ebay vs e-commerce.

Thanks for reading.