eBay vs. brand
Here in Oz at least, the top brands - at least those that people will seek out, rather than those that need passing traffic - are moving to an agency model, where the customer deals with the brand owner (facilitated on-line in the retailer's shop!) and then the retailer is paid a commission.
This means that the goods are the same price everywhere. (We can only guess the competition regulator's view on this, maybe they also realise how rapidly the retail game is changing.)
So working through the consequences for eBay -
Would the brand owner allow a different price on the net - don't think so
Would eBay get a different commission compared to a retailer - probably
Would it be more or less than a retailer gets - the retailer provides a much more valuable service, so probably less
So, the conclusion seems to be: brand owner takes all, eBay gets not much.
This aligns with the viewpoint of the brand owner - would they get an additional sale by selling through eBay? Seems unlikely.