You should not list your methods on the "not to sell" list if you feel they are rude or aggressive.
Don't try to bypass a gallerist by purchasing directly from the artist they represent. This puts the artist and their gallery in a difficult spot.
Your professional partnership could be at risk. This could also affect your ability and reputation to buy works from dealers.
Do: Request a condition report
You can request a condition report at any gallery, auction house or directly from an art dealer.
Particularly for works from the past which belong to the "secondary market"
Although there is no one-size fits all template for condition reports there are some common standards. A majority of condition reports contain a diagram.
The artwork must be noted for any marks, scuffs or tears. Here's an example condition report from Georgia O'Keeffe.
Based on the value of the item is, it might make financial sense to get a neutral third party to complete the condition reports.
It is likely that you will have to pay to use the service.