How can I create a bot that buys stuff automatically online?

The problem of buying automatically on the internet is that most of the commercial transactions have algorithms against Bot’s behavior.

If you plan to create a bot for this specific purpose, we need a few assumptions:

You have some experience with back/front end (e.g. web frameworks like Python or JavaScript) and databases (SQL or NoSQL)
You already created some easier bots like chat-bots or scrapper-bots. Find more types at: 7 Types of Bots
You have all your worflow/system architecure planned and working manually. You can use a tool like “Page Monitor” to help you with that. Download here

Page monitor extension allows your browser to monitor changes to web pages for you. It can inform you whenever a particular page changes without you having to go and check every time.

So, here is what I think that is necessary in a hypotethical scenario where this activity would be legal:

Instead of crawling the web by yourself, get your results from a site that already indexes shop pages: http://www.nextag.com/
Use a programming language to create the whole process of your bot. I would recommend Pyhton or NodeJS (for this task, I think that Node.js is preferable)
Bypass Captchas using some Image Recognition API or specialized sites like: automated captcha solving service.
VPNs to change your IP constantly (if you are planning to buy multiple times a day)
Side note: Even if you are not automating your shopping process is good to take a look at VPNs: How VPNs Are Changing The Global Marketplace For The Better
Avoid Location-Based Pricing
Make Travel More Affordable
Get Access To Better Deals
Shop More Safely
Create different shopping accounts that can be accessed throught your bot
Create different PayPal/Xoom accounts that can be used by your bot
Find a way to put it all together without infringe any laws or policies from the shops and/or governments.