Domains continue to rapidly increase in value... just because you let it expire doesn't preclude leglink.com from having value to someone else; ideal for a legal related site; it already appears, based on some cursory searches, to be the name of a service / brand.
With that said, $688 is on the lower end of what BuyDomains lists domains for so it likely not worth much ... but who knows.
View the reported sales of the week for an idea of what many domains are selling for...
http://www.dnjournal.com/domainsales.htm
Ron