The eBay fee structure can be pretty brutal for selling things that might only go for around $5, particularly after their recent fee increase. The listing costs about 35 cents per item, one picture costs another 10 cents, then there is the final value fee so now we are over 50 cents to offer one slide or photo. Then if the buyer pays with Paypal it is another transaction fee plus a percentage of the sale price, usually close to 50 cents total for one slide or photo, so now the costs are up around a dollar. Some sellers use quite good packaging and may lose some money doing so as a service to keep their customers happy. Then if you figure that only half the listings they post in a week might sell, that could be as much as $2 in expenses per slide or photo that actually gets sold.
On the other hand if they get lucky and one slide goes for $175 (like one railroad subject I saw auctioned recently), it can really make their effort doing the listings worthwhile, but that doesn't happen very often. It is actually a lot less time and effort to take a rack of slides to a train show than to sell them on eBay, and the selling expense per slide may be lower in many cases.
For the buyer, eBay is a pretty good deal because it lets you find obscure items quickly and easily from the comfort of your computer desk without having to pay for gasoline, lodging and meals to attend distant train shows. I use eBay frequently and it is a great convenience when you are looking to locate and buy items which would otherwise be difficult to find.