Im just wondering if you model or buy/downloading the props for your levels?
I prefer to model myself for experience and practicing.
I’m entirely modeling my assets by myself 
-> good practice
Some people also buy models to study how they were made. Topology, uvs, etc. =)
I also do that as well for human models, interiors and so on sometimes.
I model mine ,but i got some money from epic so I probably spend it on the market place soon
In reality the standards say you have to be able to model your own stuff for the level you design. 3D artists are there to make the more complex things like weapons, vehicles and such. But that aside if you are not employed like that, time and experience are 2 variable factors for every individual and that makes people either make their everything for themselves, use free models online or buy assets to design their levels, hire someone to make assets for the level you want to make. level designers fall into all these categories. I personally like to do everything myself so I’m not showing off with someone else’s efforts.
Edit: Plus the experience you earn is so valuable.
Both. Depends on complexity / time / costs / unique-ability
Both. Purchasing has licensing issues but if done right you can limit licensing exposure to just a few. DAZ is a good example of this. I do create the more simple things on my own with nifty UV software (Ultimate unwrap) because I can model and shape something…but UVs suck! :rolleyes: Also if you buy…buy on sale. DAZ DAZ DAZ. Once I get past the individual assets I then place/paint them into a landscape/house/building/whatever. I love doing this and it gives personal pride to see the result.
In the end…time is money…spend time painting or spend time on 3D models…the choice is yours.
For practice buy/download, for projects you’ll have to make it yourself or your team.
It’s very relative for me; If it’s something that costs more time than money to make, since I’m working on a commercial game I must value time spent on the right tasks.
Although I’m experienced modeller (14 years of Maya) sometimes I may need to buy a model that will cost me too much time to build.
I grab some from turbosquid, make what I’m capable of making when I can, and but something here on the marketplace if I can use it. My knowledge of modeling is limited, but I can sure make some nice crates when I put my mind to it. 
I guess both? There are things which do not have to be replicated, so i try to focus on things which are not available or out of reach. Started with Blender and i feel it is really nice, you get results pretty fast, but you have to follow some tutorials at first.