I did that too a couple of time but only on coins I just HAD to have, when it wasn't about market prices or economic sense anymore but just infatuation w/ a certain aspect of that coin - the bust type, the relative rarity, etc.
I am almost sure that at least 2 coins I got this way will not make my money back if I decided to part w/ them. And that's just another incentive to not part w/ them
PS - the Constantine II BEATA w/ half-length bust holding globe and spear I got from you last autumn is not one of them and although at that moment I thought I might have overspent a little for it, I still think it could commend at least 170$ at an auction just for its rarity and unusual bust type.