I feel like by the time you get a market for a specific item that item is already easily craftable smeltable so there’s no point in buying anything. Maybe I’m using them wrong?
Depends on how you define "easily craftable".
ARGUMENT #1: Deprivation of Resources
This means you (A) have to have the ore to be able to (B) smelt the alloy bars which will then allow you to (C) craft the item and then (D) repeat as many times as you need that item.
So when you need a low quantity <50 of an item like Cooper Wire for example, this is indeed pretty easy. Even 20-50 of Tin Cans or Screws for example is fairly easy. BUT also know that making Screws requires Iron Bars which requires Iron Ore. But making Platinum Bars also requires Iron Ore (which you can't buy) - did you just use up all your Iron Ore making Iron Bars to make Screws that you could have bought from the Market and thus have none to smelt Platinum Bars with that you need for a Tech? If so, your only option is to (A) wait for more ore to generate (you can buy mine upgrades to help, but you're still stuck waiting for it to accumulate), or (B) buy Platinum Bars from the Markets. Conversely, you could just buy Screws from the Market and save your ore.
ARGUMENT #2: Time Saving
Instead of 20-50 of Copper Wire, Tin Cans or Screws, let's say you need 1500 or each of these. That takes a long time. If you have a surplus of money and can buy them from a Market, you can get that Tech now rather than 10-20 hours from now, or in some cases (depends on how many Smelters & Crafters you have, how many Techs you have, etc) it could save days. So what's more important at this point in time, saving hours/days to get Techs quicker (and the benefits they bring) or conserving money to invest in Hospitals, Army Camps, Mine upgrades, etc? The answer may differ depending on your state within the level and your preferred play style.
ARGUMENT #3: Efficient Use of Smelters/Crafters (And More Time Saving)
Keep in mind that later levels require you to make items which are then used to build other items (like building Screws to build Resistors, Circuit Boards and LEDs which are then used to make Digital Clocks, etc). If you're using your Smelters for Copper, Iron and Tin, etc, it means you're not using them for Platinum, Titanium, Dysprosium, Unobtanium, etc. If you're using your Crafters for Tin Cans, Screws, Nails, etc, it means you're not using them for Resistors, Computers, Motors & Solar Panels. You only get so many of each, so use them for what benefits you the most - which likely isn't the cheaper items. Again, it comes down to whether you have a monetary surplus and whether you want to focus on getting Techs more quickly or not.
If you're like me, you probably end the level with either (A) a surplus of both armies and money, just waiting for the last few Techs to finish, or (B) exactly the # or armies required to clear the level and a surplus of money. Take that future Surplus of money and move it forward to spend more effectively as to optimize your level playing experience = my recommendation.