meh

Unfortunately SilkSpawners does not work properly anymore with Essentials shops.

The only alternative is to use these two plugins to sell spawners:
ServerSigns: http://dev.bukkit.org/bukkit-plugins/serversigns/
Spawner: http://dev.bukkit.org/bukkit-plugins/spawner/

You would setup a sign with any text you would like, for example:
[Buy]
1
Zombie Spawner
$15000

You would then use ServerSigns commands to attach these commands to the sign:
/svs setprice AMOUNT
/svs add <server> /spawner give MOBTYPE <player>

In the above example you would put these commands to give players a zombie spawner for $15000.
/svs setprice 15000
/svs add <server> /spawner give zombie <player>

To add these commands to a sign simply click the sign once you have typed in the command.

As well, players will need this permission to use the ServerSigns:
serversigns.use