Can't say about what quality they are, because I have never bought any, but I can predict what quality they might be at these prices. Keep in mind that they still have to make profit.
Their sources are most likely two: wholesale and batches of cosmetics that couldn't be sold under the brand they have been made for (e.g. bad quality, wrong proportions of ingredients, short expiration date, factory producing after hours using leftover, expired, or lower(ed) quality material, etc.). For wholesale they would have to buy really big batches, and not of the greatest quality. For the rest, they are just too low quality for normal sale.
I'm a newbie, and am also overwhelmed by the prices in brand stores, but I for sure wouldn't buy anything from Shop Miss A, even if the prices might be tempting.
If you really want to try them out, I would avoid anything that goes on or near your lips and eyes.
And definitely do not buy non-cosmetics products from them, like brushes or jewellery. You never know how fucked up they have to be to be this cheap. I have ran into a review of a brush infested with cockroach eggs. Regarding jewellery, it might contain dangerous metals, and they are open about it containing nickel.
I would rather buy cosmetics from China. They are more than $1, but it shows how much it has to cost to produce and export such a product. If they were produced in US, they would have to be much more expensive.