Short answer: Compare the price with the cover price, adjust for condition and edition, and add delivery before deciding.
Start from the cover price
Find the original price and see what share of it the seller is asking.
Adjust for condition
Heavy highlighting or damage should lower the price. A clean copy of a popular edition can sensibly cost more.
Include delivery
Delivery is added on top and depends on packed weight, so a heavy book far away costs more to ship.
Frequently asked questions
Is a lower price always better?
No. A low price on the wrong edition or a poor copy is no saving.
Can I negotiate?
Use the in-site chat to ask the seller questions before ordering.