I have come across quite a few good bargains from time to time but sourcing ammunition and components this way is not something to be done carelessly.
When I do buy this post consumer merchandise, I expect to buy it at a significant discount from the current retail price as the transaction is not without risk.
Purchasing Preowned Ammunition
Paper shotshells particularly those manufactured more than a decade ago are unlikely to function reliably. They should only be purchased for the purpose of salvaging the shot. If it is later found that they do go bang when they are supposed to so much the better.
Buying Other Peoples Reloads
I never shoot reloaded ammunition that I have not loaded myself so I generally don't buy other peoples reloads.
Sometimes I will salvage the components to use in my own reloading and will make my offer to the seller with that in mind.
Purchasing Preowned Reloading Components
I buy sealed containers of powder only. The inadvertant mixture of the wrong powders could mean disaster for your gun or yourself. Understand that the seal is no guarantee if dealing with a sharp trader looking to cheat you.
Out dated or obsolesent components can often more aggravation than they are worth. You may not be able to find usable loading data for smokeless powder that no longer is in production.
Using Preowned Components
I don't like the idea of having to take apart ammunition that I have put together. To avoid this, I always make small test lots (20 rounds or less) when trying new loads or use newly acquired preowned powder and or primers.