I work with many Cisco ASAs in the field that have been purchased from ebay then put into production. They work great, but sometimes people cannot connect, or in one instance the device kept rebooting.
This was because of the license limitation of the device.
The ASA has two main licensing modes, base and security plus.
Base allows a user limit of 10, so what is a user?
A user is any host that communicates with the “outside” vlan/interface. If you want to check how many users are in use run the command “show local-host brief”
So in this case only 2 users are counted. This number will increase and decrease as users timeout.
To extend the user limit you can upgrade to optional 50, or unlimited license level within base, or just get the security plus with unlimited.
Cisco of course has a ton of licensing options, here is the link for Cisco 5500 series http://www.cisco.com/c/en/us/td/docs/security/asa/asa82/license/license82.html#wp204305