I have purchased several $100-$200 bikes from Marketplace. They are all fantastic bikes. My best advice however is to have enough cycling experience, before you start buying, to know exactly why you want a particular bike, rather than just looking for a better bike in general based on it's brand. Sometimes it's hard to get that experience though if you don't have any opportunities to ride different bikes. In such a case I look at a $100 purchase as entertainment and education. I figure even if I only get a few rides out of it before it needs work, I've had more fun for less money than I could have had anywhere else.
If you are going to start making these used purchases you need to learn basic mechanic skills first. Get on Youtube and look into learning basic repairs and service. I enjoy working on my own bikes a lot more than I ever thought I would, plus you get to try out different modifications,and can customize something to your particular needs.