I know I am quicker to block bidders now than I was a year ago and this is simply because of
ebay's "Defect" criteria.
In the past if someone was unhappy with any item I sold I would accept returns and try to make the person a repeat customer by being very gracious and friendly during the return process. Now if they return an item because it is not as described I get a defect. If they return it for some other reason I get a defect when I refund the Paypal payment.
I sell country collections and I have had bidders push the "Not as Described" button because 1 stamp in a collection of hundreds was not the one they thought it was. I immediately block such a bidder because if they do it once they'll probably do it again.
Just yesterday I blocked a bidder because of an
ebay message exchange, he has never bought anything from me and he never will. He wrote to me saying the cost of shipping to Belgium was too high compared to his estimate of the sales price of the collection. I responded that the shipping is based on the weight of the item not the sales price and if he thought the shipping cost was too high please don't bid. He sent an irate response and I immediately decided I did not need this guy as a bidder.
If a bidder contacts me through
ebay message service with a problem I will do everything I can to make the bidder happy especially if it is done in a congenial way, but if they push the "Return Item" button first they will probably end up on my blocked bidder list.
When I'm deciding whether or not to block a person I check to see if they are a repeat customer, if they are I won't block them. I also may check the "Feedback Left for Others", but I couldn't care less if they have 300 positive feedbacks or 3000 it's meaningless. I also don't care about losing future bids from a bidder that I think has a high probability of causing me trouble. I lose several bids each week because I block bids from buyers who have Unpaid Item strikes, but I think it's worth it.