Couriers and postal services are all pretty variable, and some work better for some types of goods and locations than others. For example, in our local area, Evri are actually pretty good and Royal Mail are appalling, but in other areas the reverse might be true.
When you're looking at an item, if you go to the "Postage, Returns & Payments" tab next to the item description, it should say what postage options the seller has set up. Usually it'll show you a specific carrier, although sometimes it'll just show something like "Other 48 Hour Courier". Often, it'll show a couple of options at different prices depending on speed.
You have every right to expect them to send it by the method they show.
If they have selected Evri (or the generic courier option), you can either move on to look at another listing, or contact them to ask how much extra they would charge to use Royal Mail, DPD, or whatever. Bear in mind that eBay is a very price sensitive marketplace, and many buyers will choose whatever seller is offering the cheapest price, not neccessarily the most suitable.
Another thing to consider is that review sites are place people go when they want to get something off their chest. How many people leave a good review for every piece of post or courier delivery that arrives on time and in good condition ?
