It's really a question of whether they spot it - I also think there's a group of anti-gun types who keep their eyes peeled and put in complaints as much as possible on items.
However, there was one auction recently (I think from Australia) which had a batch of enfield-y items, which included a bolt head (I think in the uk at least you'd have struggled to buy it normally without putting it on your ticket as it's load bearing - although it's naturally down to interpretation of the law!).