Dungeon Master's Guide
For each salable item, the character makes a DC 20 Intelligence (Investigation) check to find buyers. Another character can use his or her downtime to assist with the search, granting advantage on the checks. On a failed check, no buyer for the item is found after a search that lasts 10 days. On a successful check, a buyer for the item is found after a number of days based on the item’s rarity, as shown in the Salable Magic Item table.
A character can attempt to find buyers for multiple magic items at once. Although this requires multiple Intelligence (Investigation) checks, the searches are occurring simultaneously, and the results of multiple failures or successes aren’t added together. For example, if the character finds a buyer for a common magic item in 2 days and a buyer for an uncommon item in 5 days, but fails to find a buyer for a rare item up for grabs, the entire search takes 10 days.
Xanathar's Guide to Everything
A character can find a buyer for one magic item by spending one workweek and 25 gp, which is used to spread word of the desired sale. A character must pick one item at a time to sell.