What happens if the seller refuses to sign over the domain name and social accounts after closing?

If the domain name and social media accounts were included among the purchased assets, or were the subject of an express post-closing transfer covenant in the purchase agreement, the seller's refusal to hand them over is a breach of that obligation. You generally have remedies including seeking a court order compelling the actual transfer, since what you need is the asset itself rather than money, along with a damages claim for any resulting harm — lost business tied to that online presence, for example.

If the purchase agreement never actually addressed these specific digital assets at all, your position is weaker and depends on whether they can be shown to fall within a broader description of purchased assets already in the agreement, which isn't guaranteed. This is a common gap worth catching in future deals by naming digital assets like domains and social accounts expressly, rather than assuming they're obviously included alongside more traditional business assets.