Sewing, Repairing and Reducing Waste

I buy most of my clothes second hand, the quality is much better for the price but when I look for something precise, too replace something wear out for example, rather than a new piece for my wardrobe, it take me a lot of time to find what I look for. Meanwhile, fast fashion brand sells lots of clothing that seems exactly what I need except that the quality is fast fashion quality :(

Is there any general advices you have to improve bad quality clothing?

Here is two specific examples that really disturb my shopping experience:

• T-shirt that have not T, because the cloth piece under the arm has been removed. The t-shirt looks fine, folded or wear with your arms lying against your body but as soon a you raise them, in a T, or high to reach some high shelf, the whole t-shirt raises with them
• Long sleeves than a just long enough to keep your arms lying against your body. If you go for a crazy move, like using your hand, you hand up with almost 3/4 sleeves.

What do you think? Is there anything that can be done to improve these type of badly cut clothes? What about all the others? With loose stitches, bad made buttons... You know what fast fashion looks like.