I agree with Bohrium. It depends on which objects you buy and which you make yourself (or hire an artist to make specifically for you), it depends on your specific project, your art style, etc. For example, if your game has a realistic art style and you need a model of a generic real-world weapon, say an AK-47. Buying that online is perfectly fine because chances are that it would’ve looked pretty much the same if you had modelled it yourself - a realistic AK-47 is a realistic AK-47 (okay, people who’re into guns would probably tell me that that’s not the case but you get my point), there are only so many ways you can model/texture it. Same goes for realistic, generic materials like rock, wood, basic metal, etc. Especially when you’re working on a one-person project, it tends to be way more efficient to buy a lot of your materials, rather than make them yourself.
What you should create yourself is stuff that’s specific to your game. So, say you’re making a fantasy game and your main character explores the ruins of a fictional, ancient culture with its own fictional architectural style, script, etc. In that case, you wouldn’t want to use some random fantasy pack you got off the internet, you would want to make sure that these assets actually fit your world, story, etc. and the only way to do that is to make them yourself (or to have them made specifically for you). So yeah, in other words, anything that’s so generic that it wouldn’t make a big difference whether you used something pre-made or made it yourself, you can buy. Anything that’s specific or easily recognisable as an original piece of work on its own, you should probably think twice about.
Things get much more complicated if you’ve got a stylised or cartoony art style, though, because then it becomes a lot more difficult to keep your art style consistent. You need to make sure that all the assets you buy fit in with your game’s style (which is much easier with realistic games because there tend to be more realistic assets out there than stylised ones).