Complex sentence: While many accounts were created by fans during the early registration phase, many users created multiple accounts and sold them for high prices on eBay, despite being warned by the Pottermore blog and being assured that the website would remain a free site.
Split into simpler sentences: While many accounts were created by fans during the early registration phase, many other users created multiple accounts and sold them for high prices on eBay. This was done despite the warning given by the Pottermore blog and the fact that users' were assured that the website would remain a free site.