Those are two niches notorious for having virtually zero margins, and being almost impossible to source competitively for eBay selling. Everybody somehow seems to start out trying to get into them, but it's really virtually impossible to do so profitably.
I know less about the fashion stuff, but with regards to electronics and media, you have to bear in mind that major retailers are selling the stuff for profit "margins" that would be more appropriately called profit "shavings." And they're buying in six figure quantities, so it's a given that TigerDirect, Best Buy et al. are getting better prices than you can ever hope to get.
To get an understanding of the business side of electronics, look at the trend in retail
pricing in electronics over the last decade, vs. the rate at which new in-demand products have been introduced. Electronics as a business is a classic case of loss leader products -- all of the profit is made by the manufacturers. Retailers use electronics to get you into the store, in hopes that they can sell you $10 cords and $30 printer cartridges and other accessories with real markup to them. They practically give away phones to sell phone service (Amazon paid me $50 to take the one and only cellphone I ever bought -- I actually ended up $50 in the black after rebates, just so they could get a slim commission from T-Mobile for signing me up for service.)
Chinese-made stuff can have some decent margin to it, but you have to be able to identify and sell to the market for unbranded goods, people who want the look and function but don't care about the panache of brand.
I'm not trying to be discouraging -- just saying that in order to succeed in any niche, you really have to step back and look at it as a whole, not just as a matter of price but presentation, upsells/accessories, lead capture, etc. Too many people look at stores selling a given product for say $200, assume the wholesale is $100 then can't figure out why they can't find a source under $180 anywhere. Price is not really the driving factor in profiting in electronics.
"Failure is not when you fall down. Failure is when you don't get back up."