Most games are not entirely created by one person.

For example, music and sound creation are completely different talents to development of software as are artwork and creative writing used for plots and background information.

Admittedly you could do all this yourself but are you saying you could do them all yourself WELL and within reasonable time frame?

Don't be ashamed to use other people's talents to finish a project to a good standard.

I also believe one cannot be talented in every aspect of game development.
Games mostly come from teams,, be it 3, 10 or hundreds of persons.

This is true now, but 20 years ago it was possible for one person to make a best selling hit so long as they had a small amount of musical and artistic talent and a lot of in depth programming knowledge of at least one popular platform. Remember people like Jeff minter for example? This was because the player expected much less from the computer in terms of music and art, and the computer was less capable.

Unfortunately those days are long gone in the current age of cd quality sound and HD 3d graphics

like a painting or a piece of music

Sure, but does an artist have their own art gallery where they display their art to the public? Does a musician have their own concert hall and orchestra?

Very few things in modern life are achieved completely alone, it is human nature to work together.

So should I buy some assets, or is that considered cheating. I'm not sure how I feel about it personally. I'd love it to be 100% my creation, but it would definitely speed thing up.

Cheat all you need to make a game. It's the only moral thing to do (no player will care what percentage of the product is made by you, they just want to have fun).

If you go the purchasing the sprites route, I'd say try to get them all from the same artist, or put a lot of effort into making sure the style matches up with them. I think you could potentially waste a lot of money and just end up having all your assets clash together stylistically. I'd also suggest waiting until your game is almost finished, as (if you're like me) a good portion of your games may never see the light of day (you also mentioned they'd be background sprites, so they shouldn't be too hard to shoehorn in at the last stage).

No use purchasing assets for your own learning process, or even your friends and family (they'll forgive your placeholder or self-made sprites :P). If you're not planning on releasing publicly, no harm in brushing up on your art skills and doing them yourself.

I'd also recommend something like opengameart.org, as maybe a middle step, giving you some decent and free assets in the meantime. When you're ready to release, you can then maybe have a better idea of what you'd like to spend on professional assets.