Having an FFL myself for a while in the past, I can tell you dealers do not make much on guns. Distributors are usually pretty close on wholesale prices.
Large purchases allow for better shipping costs for the dealer. As with many online sellers who have lower prices they charge than brick and mortar stores, they have to sell a huge volume to make money.
If you buy online, make sure you figure in all fees, not just the cost of the item. Shipping, sales tax, transfer fee, credit card fee (many Gunbroker sellers charge credit card fees, around 3%).
Buying in person allows you to look at the item to check condition, stamps, etc. and you get it that day. I am in no way bashing buying or selling online. I have done both myself. I am just pointing somethings out.