It's simple economics of supply and demand. I must have been active both buying and selling on ebay for 20 years (can't remember how long, but it's a long time - probably soon after it began). Even back then you could sell things close to, or over, shop prices.
The thing is, that some people don't have a good range of shops nearby, or don't have time/transport to go shopping, so Ebay is convenient for them. There've certainly been a few times when I've sold things for more than I bought them new - things that were still widely available in the shops at the time, but obviously, the buyer couldn't/wouldn't actually go to the shops themselves to buy direct.
Nowadays, with widespread internet shopping, that's less of the case, but we're now in the realms of shops not selling what people want anymore, so sometimes, buying "as new" or "BNWT" is the only way people can get specific types of item. Eg, my 82 year old mother liked M&S cardigans - they don't sell her style anymore, so for the last few years, we've been buying from ebay, either worn exc cond, or BNWT (bought but left in wardrobe etc) - prices being very similar to what they'd be in the shop IF M&S still sold them! I bought a lot of Joules stuff a while ago (socks, blouses etc) when I won a voucher in a competition - I basically just bought stuff because the voucher was about to expire - I never really wore much of it and most was still BNWT - I've now sold it all and every single item sold for more than it's label price - that's simply because the designs/styles weren't available in their shops anymore.
Perhaps the shops need to pay a bit more attention to what people actually want to buy, and perhaps they should put more stock in their High Street shops so that people can go and buy it. Ebay is getting expensive due to lack of supply by the shops - the shops who are complaining about lack of customers!!