Its probably a unfair comparison as you rarely see the sellers talk about when they are earning loads.
Reselling works if, as the OP says, "You buy cheap and you sell high"
Reselling doesn't work if you are selling the same items that EVERYONE has got from Tesco when they reduced their stock. You need to do as the OP does and find items for 20p and sell for £10.
You are 100% on the money on that one the number of times I've seen offers go up on hotukdeals which everyone pounces on and then they end up on ebay and the prices just drop and drop and drop.
A good example of this was the SNES Minis last year which didn't go out of stock like the NES Minis did and people were putting them on ebay for like £100 after buying them for £60 odd and they couldn't then shift the stock to turn a profit because suddenly black Friday week appeared and everywhere was selling them for £43 and the bottom well and truly fell out!