The best way to uncover a reserve is to place a bid....if your bid doesn't make the cut then you can walk away satisfied. It costs very little in terms of time or money to place a bid.
If your time is too valuable to bid on a $3000 gun with an undisclosed $5000 reserve it just tells me you are not a serious buyer.
Think of all the years before the internet guys traveled to gun shows (some still do) in hopes of finding a righteous gun. That took dedication. Clicking a few icons is a cakewalk.
Again, your issue is with Sh**Head sellers who abuse a reserve, undisclosed or not.