I have fortunately had just the opposite experience over many transactions for both inexpensive stuff and really expensive stuff. I had two transactions this week and both were from USAM and both went very well with extremely polite and friendly people - one buyer and one seller.
In addition to treating others as you would want to be treated, one other suggestion that has served me well is to be willing to walk away if you are not getting prompt responses, if the other party seems rude or inattentive, or if things just don't seem right.
In addition to treating others as you would want to be treated, one other suggestion that has served me well is to be willing to walk away if you are not getting prompt responses, if the other party seems rude or inattentive, or if things just don't seem right.