Army surplus is the material that the army no longer needs because it's outdated, replaced or the army simply has too much of it. This equipment could be camping necessities, MRE's, uniforms, packs or any of a thousand other items that the army uses on a regular basis. These items may then be sold off to civilians, who may then sell them through army surplus stores, dedicated websites or through web auctions.
Acquire army surplus. This can be done in person at government auctions or through the Internet through government liquidation auctions. Websites such as govsales.gov or govliquidation.com are good places to start if you're looking for army surplus.
Open a store to sell your surplus. You'll have to get a business license from your local board, in addition to getting a physical location. If you intend to sell military surplus weapons and ammunition then you'll also need gun licenses for your store in addition to regular business licenses.
Sell army surplus via the Internet. This is another option that you can take instead of getting a physical location to save on overhead costs. Auction sites such as ebay.com and others will allow you to sell a variety of items, including period bayonets and knives. You won't be allowed to sell firearms or ammunition though, so those sorts of sales will need to take place via other channels.
Make sure that your army surplus is genuine issue, and not an imitation.
Certain military items, such as headgear with insignias, badges, buttons and other authentic items that may be used to impersonate an actual serving member of the armed forces, are illegal to sell or buy.