Excellent question. We pull comps from two sources, primarily. The MLS and the county recorder's office. The MLS is comprised of closed buyer transactions sold using a real estate agent that is a member of the MLS. If you are looking at a report that was prepared using the county records then you will see a mixture of comparable sales that were facilitated by either a Realtor or another entity. Trustee Sale records are actual sold properties too but can be defined as distressed sales and considered as such in an appraisal (usually and hopefully). If you have a list of recent closed sales look at the new owner of the property to see if it is a bank, financial institution or private party to help determine what type of sale it was. Also check the tax stamps paid or transfer tax if you can. If it is zero then it is not considered an "arms length transaction" or valid comp. There are other signs to look for but I don't want to bore anyone further with my explanation.
By the way, a trustee sale can also be purchased by a valid "buyer" too.