There are multiple plugins to manage and serve ads on your WordPress website. I don’t know the Reddit system, but here are three solutions I have seen used by clients who let advertisers purchase ad space directly.
- Simple Ads Manager – like it, but not always trustworthy in the latest updates
- AdRotate - ok for most self-serving setups
- OIO Publisher – unnecessarily complicated
I ended up developing my own "Advanced Ads" plugin, but it does not yet have an interface where advertisers can purchase ad space directly.
DFP is a full-fledged ad server solution. Most of my clients struggle with it, because the learning curve is high and it very complicated too. There are very few cases in which I would recommend it. Even if you use it, it often makes sense to manage their ad tags through an ad management plugin in WordPress too.