What were their terms? There are certain procedures that protect the seller and the buyer.
When I buy on ebay, I first look at their terms. I also look at their feedback....not just the percentage of positive feedbacks but at the actual comments and what they usually sell and ship. If someone has a high volume of business (say 1000 auctions in one month) but just 8-15 of those auctions have negative feedback each month, then I usually don't bid on their auction unless I think the negative feedbacks are frivolous.
When I bid on an item, if it requires immediate payment, I make sure that I have the immediate payment to give to them. I also don't leave a negative comment until I have checked out all other options.
For many of the sellers, this is their business. Also, life happens...someone could end up in the hospital and not be able to handle their items that they are selling. Some could have just lost a loved one. I try to give them the benefit of the doubt. I would probably leave them a negative comment if they hadn't responded in 3 Weeks, but I would never leave a comment for them not responding in 3 days.