Will a Bell SIM card work in a Virgin Mobile iPhone 5?

Purchasing a used iPhone 5 that's locked to Virgin, I've heard that Bell SIM cards work on Virgin phones, but want to make sure.

You do want to test that out first. While it is true it should be compatible carriers can and do lock certain SIM IDs (known as UICC IDs) to a particular brand or device. For instance Bell owns Virgin Mobile in Canada; but they clearly target a different market segment. It is very likely they have made customizations for Virgin customers and for Bell customers.

In summary, it is technically possible BUT you should do and actual check/try-out before committing to the purchase. The caveat is that carriers can and do lock SIM Cards based on the SIM ID. (The UICC ID allows to distinguish if the SIM is Bell, Telus Virgin, Koodo and so on based on that they can reject the network registration attempt when a non-allowed SIM/device combination attempts to gain access to the network)