HGG Community Forums

From the FAQ user guide

Why does it say my account is associated with someone elses accout?
Internet Service Providers are given a block of numbers called IP addresses that are rotated for their users. Each time a user logs on, their computer is given one of those IP addresses. When that user ends the session, that IP address is released for use by another user. So, if your Internet Service Provider is a large, national one, you can become associated with another user somewhere else because you both have been randomly assigned the same IP address at some time.
Unless you are paying to have a stable IP address, you will frequently have a different one each time you log into the Internet.

There may be a similar mechanism involved with phone access to the internet. If both you and the other player have the same cell phone carrier and they use a rotating block of these numbers, you could easily have become suddenly, randomly associated.

Once your accounts are "associated" unless both of you have Premium accounts you will not be able to purchase from each other. If you both publically post about the horse someone may step up to be a middle man for you :)