Buying Individual LEGO Parts
Whether you've misplaced a part from a set, or you're looking to add a new extension to your Lego house, you've come to the right place! You've got a few options for trying to find those parts you need. Brick Owl is a marketplace comprised of many different Lego stores, so there are a few ways to buy parts on Brick Owl.
Buying from one store
If you only want a few items, you can search for an item in the Catalog at the top of the page. You can then click on an item, and scroll down to see all of the available stores. You can then click through to that store, and take a look around the store, adding items to your cart in the store. You can then checkout directly in that store.
Buying from multiple stores using the Catalog Cart
If you've got more than a few items to buy, but less than 50 or so, you can use the Catalog Cart. When looking at items in the Catalog, you should see turquoise cart buttons. You can click on those to add different items to your Catalog Cart. Once you've got what you need, you can then to go your Catalog Cart, where hopefully you should see Brick Owl's best combination of stores to purchase the parts from. Assuming all has gone well, you can then click on the green button on the right to add all the items to Store Carts. Store carts are the carts within the actual stores themselves. You will then need to go to the individual stores to proceed through checkout in those stores.
Buying from multiple store using a Wishlist
Lastly, if you have a lot of items to buy, the Wishlist tools give you the most flexibility and control. Once you are logged in, you can search for items in the Catalog. you should see a Wishlist tab on the item, which you can use to add items to your Wishlist. once you've got everything you need in your wish list, you can then use the Wishlist buying tool to find the best combination of stores to order the parts from. Using the tool, you can then add the item to the cart in those stores. You will then need to go to the individual stores to proceed through checkout in those stores.
Searching by name
If you have an idea of what the part is called, that's easy and you can just search for it at the top of the page. Alternatively, if it has some studs on it, you can search for the amount of studs, for example "Brick 2 x 4".
Searching by ID
If you are trying to replace a part, another option is to take a look at the back of the instructions, where you should find all of the parts along with their "Lego Item Numbers" which can easily be search for. Each of these Item Numbers is colour specific so will take you straight to the correct part. Sometimes if you're lucky varies even an ID number printed on the bottom of a part
Lookup an Inventory
If you happen to know a set that the part comes in, if you search for the set, you can click on the "View Inventory" button and find the part in the set that way
Lastly, if all else fails, just browse around some of the Part categories and hopefully you'll find what you need