It may sound too basic to advice jewelry traders to always bring their own loupe whenever they are going to buy gemstones. However, the impact can be huge if you forget to do this basic method.
Quite a while ago, my supplier shows me one Ceylon blue sapphire stone size about 3 carats with obvious crack under the pavilion when observe under the loupe. I personally believe he know about gemstone buying better than me, but I asked him how can you make this type of mistake before you purchase this stone. He said at the time he purchased this stone, he did not bring his own loupe with him but instead using his dealer loupe instead.
I further asked whether he can re-cut this stone, he said the weight will become under 3 carats size which will make it hard for him to resell. Since that day, I personally bring my own loupe whenever I going to purchase any type of stone.