On the alcohol, no matter where in the USA you are at you will need to have a Licensee to sell alcohol, no matter what. Either keep it for yourself, or give it as a gift to someone.
On the guns, different from state to state. For the most part you cant sell a gun to anyone other than a licensed gun dealer. However before you do so, you should check to make sure that the gun A: Is not stolen, and B: that it has not been used in a crime. It comes back clean feel free to sell to A LICENSED GUN DEALER, if it comes back unclean, legally you are suppose to turn it over to the authorities, and depending on where you live, they could seize the entire locker you purchased as evidence.
Now for your taxes, if you are running as a business do make sure you get all the business license that are required by your city, county, state and of course federally (IRS). Keep all receipts, from where you purchased, what you sold, gas, storage, anything and everything that you can deduct from the taxes that you will have to pay. On the IRS website they can tell you what you can and what you cant use for tax deduction, and how much of it you can use. Now however, if it is a hobby, where you dont make that much money from it (ask a professional in taxes to check) then you could be ok from not doing any taxes. Only reason I know this is my Grandmother started a business doing arts and craft type sales, after 6 months, the IRS sent her a letter stating that she does not have to file on the business because she was not making alot of money. She was considered a hobbyist. But like I said do check with a tax professional, you can even ask the IRS themselves, they offer free tax advice.