For a start its £2.80 GBP standard postage on Amazon, sellers don't choose (unless its fulfilled by Amazon) okay pedantic bit over.
A lot of the pricing on Amazon is automatic there are software systems that review and chase prices (usually down hence many of the 1p books, but also up).
There are also people doing the pricing who have no idea about books its a warehouse job to them so they sometimes have a Hail Mary price throw it high and see what comes down if people buy great, if not well its just another book sat on a shelf.
These people obviously don't have downward chasing price software activated on these items.
In addition many of these operations don't actually have ANY books on their shelves they price high and know where to buy low hence drop shipping you end up with a book shipped by someone other than the seller you thought you were buying from because the seller never actually saw the book.
So they have no books tying up capital on their shelves they just type (or steal as has often happened with my own) descriptions of books stick them on with an inflated price. There is no cost of purchase they are standing out of until the sale is made.
I also think many people are suspicious of penny books on Amazon. (sometimes rightly a real variety of quality can end up being delivered), and go for higher prices and generally get better quality, packaging, service and reliabiility than some of the 1p sellers.
Some are legacy prices for when books were not easily available and comparable but since then many have come on the market with the big resellers hoovering them up. The original high price gets left on because the sellers don't review often.
Some of course are just con artists hoping for a sucker.
But most aren't they are just out of touch inept or unlucky.