How Marketplace displays photos
Facebook Marketplace shows listings in a grid of square tiles, mostly viewed on phones. The first photo is cropped to fill that square, so the edges of a wide or tall photo may not appear in the grid. A square image such as 1080 × 1080 px shows exactly what you intended and looks sharp on modern phone screens.
When a buyer opens the listing, Marketplace shows the photos larger and closer to their original shape. A portrait 4:5 photo (1080 × 1350 px) uses more of the screen at that point, which is useful for furniture, clothing, and other tall items.
Make the first photo count
People scroll Marketplace quickly, so the first photo needs to show what the item is at a glance. Photograph the whole item, in good light, against an uncluttered background, and fill most of the frame with it. Move nearby objects out of the shot so buyers do not wonder which item is for sale.
Local buyers also want to know the condition before they message you. Add photos of any wear, the brand or model label, and the item in use or assembled. Photos that answer common questions mean fewer messages asking for more pictures.
Using the same photo for ads and shares
If you share a listing to a group or use the photo in a Facebook ad or link post, a landscape image of 1200 × 628 px is the common size for link previews and many ad placements. Selecting both the square and the link size here exports each photo in both shapes at once, still for one credit per photo.
Practical tips for phone photos
Wipe your phone's camera lens before shooting; a smudged lens is a common cause of hazy listing photos. Shoot near a window rather than under a ceiling light, and hold the phone level with the middle of the item to avoid distorted shapes. Photos taken this way need only a quick resize and crop before they are ready to post.