You need to define 3 things:
Bike Style
Budget
Your Bike Size
Bikes come in many shapes and styles. The 3 main categories are:
Those are just rough categories, there are a lot of specialized categories like Gravel Bikes (Road bikes that can go off-road). But those specialized bike styles tend to be harder to find used.
Prices fluctuate quite a bit throughout the year. Bikes are cheapest in the Fall/Winter and most expensive in the Spring/Summer when you should expect to pay the higher end of those ranges. Those are very rough estimates. Check the Tips section below to help maximize your dollar!
(I don’t have much experience with Mountain bikes)
To get your bike size, you need to measure your inseam and use an online calculator to get your bike size
Find a size sticker on the frame (S, M, L, XL, etc...). You can find the exact size on the manufacturer’s site
Ask the seller for their height and how their bike fits them.
Ask them to measure it. It’s the length of the seat tube
As a last-ditch effort, you can estimate the size of the bike visually based on the length of the inside of the headtube
Extra Large
Extra Small
The key to finding the best deal is to check the marketplace listings as often and as fast as possible. The best “unicorn” deals don’t last more than 30 min after posting.
A great way to rapidly search is to bookmark all the search result pages of all the marketplaces and put them in one bookmark
Middle mouse click on the folder to open all of the bookmarks in tabs
Check the most recent postings in each tab and close it
Repeat it as often as possible
Keep a list of requirements in your head as you’re browsing to quickly eliminate listings you don’t want.
Requirements like:
Message the seller ASAP. Deals don’t last! Always call, text, or email the seller if you find a deal. Schedule a visit as early as possible. You can always back out after checking with a friend or finding out more about the bike
Don’t bother with postings older than a week. They’re either not a great deal or the bike is already sold and the posting hasn’t been updated