They don't look exactly like first class stamps, but many people will not distinguish the difference:The one tell-tale sign sign that standard bulk rate was used is the inclusion of "Or Current Resident" after the name on the address label.

Do definitely choose a property type, and make sure you do it from the detailed property types>residential, otherwise listsource is unable to distinguish between a detached SFR and a Condo.

You have to distinguish between properties a wholesale owns and has the right to sell and properties a wholesaler does not own and only has a contract to purchase therefore only the right to assign their contract or in essence transfer the contract rights to you. 

@Michael Quarles how do one find out or distinguish sellers from buyers from the absentee list?

When printed on a yellow legal pad with the right print settings its nearly impossible to distinguish yellow letters printed on a computer vs hand written letters.

I'm just a small fish in a really big pond, there are some very large operators in my market, and everyone is getting mailed to by someone.So apparently people do also distinguish between mailers and make choices based on what they see and read.

So I think a wholesaler calling oneself an investor isn't quite accurate.  

So the reason there are different colors is to distinguish them from "spam" mail.