Work through it in this order, because the first two are free.
1
Bump it. That puts it back at the top of the newest listings as though you had just posted it. One bump every 30 days is free; after that it is 99p.
2
Change the cover photo. My listings shows views and saves per listing. Plenty of views and no saves usually means the price; few views usually means the cover photo or the title.
3
Turn on offers, or drop the price.
4
Then, if you want to pay: a Highlight lifts your card above the ones around it for 7 days (£1.49), and a Boost puts it into the labelled Boosted slots on the Feed, the home screen and Chats (3 days £1.49, 7 days £2.99).