Many buyers read guides or reviews of a product before buying on eBay. They are useful sources of information for buyers, especially when buying unusual or niche products. For sellers they are also useful.
You probably sell in one or more niches. While you may not realize it but by specializing in one area you become an expert. You deal with the same product a lot, unlike a buyer who may only by one now and then. The buyer may have questions that you have come across many times. By writing a guide covering common questions you can answer those questions for many people. But selling on eBay is not an altruistic activity. What is the benefit to you the seller?
Firstly, you are showing your buyers that you are an expert, you have answered their question. Would you be more willing to buy from an expert or someone else? Especially as the seller will have a link to his shop in the guide (won’t you?).
Secondly, you are part of the eBay community and engage in it. You read the forum, some people are more involved than others, they have lots of posts. How do you feel about them? They seem ‘part’ of eBay, more solid, more reliable because they spend time on the forum. They are credible, as is the writer of a well written guide.
Thirdly, to the right of the guide some of your listings will be displayed.
The same applies to honest reviews of products. If I am thinking of buying something and I read an honest review am I going to be more or less likely to buy from the person giving me good information?
Be creative when thinking about the guides and reviews that you write. If there is a new product then that is an obvious subject for a review. Also, with a new product in an existing line there will be changes from the previous model. Is there scope for a guide helping with the changes? If you sell wild bird seed, someone has to, can you write a guide about attracting more birds into the garden? If you sell compost bins can you write a guide about the best uses of the compost they make? If you sell baby items or clothes how about a guide about how quickly babies grow, how parents can survive the first six months of broken sleep etc. what to expect when they (the babies not the parents) start teething. You get the idea.
Writing guides and reviews is a good way to build your credibility and bring more sales. It is a one time effort for a long term gain.