Masquerade! Paper faces on parade. Masquerade!
Sorry, but this is absolutely impossible to enforce without screwing half of the active userbase over. Users access Gaia from many different IP locations with the same identification, some being universal IPs used by libraries, schools, college/university campuses, recreational centers, and the list continues. People can easily change their proxies any moment they wish. You have family members and roommates using the same IPs for their place of residence. There are plenty of users that lurk only in Flash spaces, Guilds, or just lurk the forums without posting for the majority of the time, making their post counts extremely low. The opposite can be said about mules since there are many that are used for RPs or used as secondary "main" accounts that users enjoy breaking out a few times a week. There are even users that take long breaks from Gaia for one reason or another, accounting for their odd lack of posts between X day of X month of X year and Y day of Y month of Y year. Just because an account may only have a few cheap items on it means nothing as new users join every day and there are users who prefer to have a very small inventory of cheap clothing because they are not worried about appearance nor how many items they can accumulate.
There is just no way to weed out the mules over legitimate individual accounts that share the same things you listed as what determines an account being a mule. It is impossible for any site to do that unless they have the psychic ability to tell who is who and what is what.
Hide your face so the world will never find you.