I've seen SSL resellers everywhere (such as namecheap for several of their options). How are they reselling? What are they reselling? Are there any security issues with them?
How are they reselling?
They typically use their CA's API to request signed certificates on your behalf. They differentiate themselves by offering different pricing, different UI, etc.
What are they reselling?
Certificates that are signed by a public, trusted CA.
Are there any security issues with them?
Not any more than if you requested the cert from the CA directly.