It really depends on the quality of the accounts and how much time you want to invest. If the accounts are well-aged with good karma and a natural posting history, they can be quite valuable for things like community engagement, driving traffic to content, or supporting marketing campaigns in a subtle way.
Selling them can give you quick cash, but in the long run they might generate more value if used carefully for promotion or niche discussions. Of course, it’s important to follow each platform’s rules and avoid aggressive promotion so the accounts stay active and trusted.