I turned on Filter Cookies under Security > Firewall and I was unable
to sign-in to ebay, plus had another site that had issues.

So if you have DD-WRT v23 SP2, and ebuy isn't letting you sign in
[keeps looping back to signing in] look at Filter Cookies...

Now why would DD-WRTs cookie filtering kick back ebuy's cookies?
Tunes to the key of "X"...