If you want help picking out a bike, post your height, what you will use the bike for, and a link to your local craigslist.
>I want to buy a new bike. What should I watch out for?Don't buy Wal-Mart garbage.
Don't buy department store garbage.
Beware of amazon and alibaba garbage.
>Should I buy from Bikes Direct?If you are clueless enough that you need to ask, no. If you have no mechanical ability, no. If the alternative is walmart, maybe. Ask first.
>I want to buy a used bike. What should I watch out for and where should I buy?Craigslist is good for old bikes.
Pinkbike.com/buysell is good for used modern mid- to high- end mountain bikes.
Ask in /bbg/ if uncertain.
Be aware that a used bike will need some repairs and adjustments. The most likely parts to need replacement are tubes, tires, brake pads, and chain. Chains are a wear item. Inspect all of these things as well as sprocket teeth, brake performance, shifting performance, and check bearings (bottom bracket, headset, and wheel hubs) for play, and check wheels for wobble. If there is a lot wrong with the bike, you may have to put a lot of money into it for parts, and money or time for repairs.
>What size bike should I buy?The right size.
https://goodcalculators.com/bike-size-calculator/ 