This guide is focussing on a very specific area of trading, the buying of killstreak kits.
On nearly all of the commonly used tf2 trading websites it can be hard to find a kit for a specific weapon given the multitude of options available. I will show you the most effective ways to the kit you are looking for on backpack.tf, tf2outpost and scrap.tf
It also worth knowing that this technique works for collector's chemistry sets and strangifiers.

Arguably the most commonly used website, backpack.tf requires a convoluted method to find kits for a specific weapon.

Step One
Open backpack.tf, it should appear like this

Step Two
Hover over the "trading" tab at the top of the page, then select "classified listings"
Step Three
Click on the ellipses which is located to the right of the search bar, this brings up the advanced search options

Step Four
The advanced search options will appear, type in the name of weapon/hat you are looking for a kit for

Step Five
After you have selected the item you want select the drop down menu which is placed directly below the item search bar. You will then need to select the middle option (find recipes where the output is used on this item) and search.

Step Six
All the kits available for your desired item should now be listed for you, select which one you want and purchase it.

Outpost is another widely used trading website, however the ability to search for kits is much more limited thus I will show you the most effective (but not 100% succesful) method for finding kits.

Step One
Open Tf2Outpost

Step Two
Select the search option (found on the left hand side)

Step Three
Add BOTH the weapon/hat and its kit to the search

Step Four
Check the box below the search which says "match all items not just one" and search

Step Five
Look through the listings and select the one you want to purchase

Other things that may be helpful

• Below the "match all items not just one" there is an aditional information box, this allows you to search for words in a trade's description. This means you could for instance search for "medi gun kit" and have an increased chance of finding said kit.
  
• In some cases you may just have to search for a professional kit and trawl through all the listings to find ones that match your desired weapon/hat

Scrap.tf is prehaps the most popular/widely used automated tf2 trading service and is quick and easy to use. Note that scrap.tf does not currently sell collector's chemistry sets.

Step One
Open Scrap.tf

Step Two
Select the "banking" option and then select "buy killstreak kits"

Step Three
You will be directed to a bot's inventory at which point you can use the search feature to find kits for your specific weapon

Useful Information

• Scrap.tf currently operates two bots so make sure to check both if the kit you are looking for does not appear in one of the two
  
• Scrap.tf has a relatively high mark up on its items so be sure to shop around before hand to get the best deal
  
• As stated previously scrap.tf is completely automated and even allows you to pay in items from you inventory rather than pure metal/keys!

This guide was written by me and me alone!
I have no affiliation with and of the afore mentioned sites, I just use them for trading!
If you have any questions,comments,criticisms or information you would like added post it below

One final note, a close friend of mine makes awesome electronic/dubstep music which you can check out here: https://soundcloud.com/ryan-matkin
This guide took me a few hours to write so I'd be grateful if you could spare a few minutes to take a look at his work.

Thanks
-Knight