How to keep track of your inventory?
Before starting off with an inventory, ask yourself if you need it. In case you intend to trade and sell pieces, it is a requirement, but as a lego-fan, you probably don't need it.
Brickstore
Great software to manage your bricks offline, free to download : Download (brickstore.dev)
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BrickStock and Bricklink
Managing a personal inventory is not difficult : you have Brickstock which is a desktop-software that allows you to do that. It has some nice features to manage your lego-inventory. You got images for every lego-piece in their tool. You can manage buying rates and selling rates for each piece. There is a single disadvantage : the colour coding does not match the official lego-colour-namings. For example "dark bluish grey" as opposed to "dark stone grey". Both are the same colour, but they are called differently in BrickStock. You can easily get used to it, but it requires some time before you get there :-)
The complicated part is to get your inventory online into Bricklink. You can import XML into your online inventory. However, it is not possible to correctly export an XML from Brickstock. I guess there must exist some nice tools, but I haven't found them yet.
Brickstock allows you to import orders you placed and - if you are a seller - orders you received on Bricklink. But, you need to be careful when buying in foreign currencies. I manage it as follows : in configuration I set "use local currency" and set 1$ = 1€.
When an order is placed in euro's will be shown in euro's after import (not with $ sign in front of the prices) and the right prices are downloaded.
When an order is placed in a foreign currency I need to do the conversion to get my prices into euro's. Brickstock allows you to convert the prices for selected items. Suppose my items are listed into USD, then I lower the price with 15% as 1eur=0,85usd. Doinig so, allows me to use my own currency for every item in my inventory.
Brickowl API
If you're a bit familiar with IT, it is a piece of cake for you to develop an a tool that is able to use the API of BrickOwl. All pieces ordered can be downloaded via the API. Disadvantage : unit prices are in the foreign currency. Good news : The use of the API is for free !
BrickSync
For those using BrickStock and Brickowl at the same time, it's not easy to keep track as you sell on two different platforms. BrickSync allows you to keep track of your inventory an synchronize between two platforms.
Excel or custom DB
Of course, there is Excel and other custom databases. An Excel-sheet containing 100's of items with different colours ; not my favourite way to manage a lot of data. For those who still choose for a custom database : you might have a look at the following : https://rebrickable.com/downloads/ It contains a few listings and tables that allow you to start building your own custom database. Also they offer the use of an API.
I'm sure there are many other ways to manage your lego-inventory....