In writing good guides you inform all good Ebayers about how to sell well and how to buy effectively and not get conned. Of course you can write guides about whatever you like, but it helps if you write tips for sellers about how to treat the buyers, where to get stock, how to make stock, how to repair stock and how best to avoid negative feedback; tips for buyers about what is a good product (of course this could also be review), the tricks on getting cheaper products, what could be obtained free if needs be and what is dodgy stock or a scam.
In warning people of scams you need to think that you are basically warning Powersellers-that is, constant users of the site. It is no good saying things like "Do not use Paypal", "Do not post abroad" or "Do not post heavy items" because the seller is basically doing what the seller wants and basically following the money. Maybe suggestions could be made. You should either name individual scammers or not mention anything. It's no good saying "Beware of sellers from Trumpton" if you have had bad experiences of someone from there. Of course there are obviously countries with a history of scams, but again, names are preferred.
You should not use a guide as a way of selling your own stock or service.