I vote for Craigslist. If you are not in a big rush. You have to browse it regularly for a few weeks to detect a bike fitting what you are looking for, from a legit, reasonable seller. I prefer steel. I believe it is a pretty big challenge to look over a carbon bike, and a bit challenging to look at aluminum. I once looked at a carbon bike, and the carbon fork had layers obviously separating. The guy said it was just fine. What??? So, yes, you have to have some ability to judge condition, but keep in mind that the bike shop can deal with a whole range of problems. Craigslist has the best chance for someone posting some bike far below its going rate. You have to browse steadily, like daily, because if a great bike at good or super price gets posted, it wont be there long.

Second is to get on the facebook of a local biking group. Lots less stuff may be posted, but it is generally populated by knowledgeable, decent people.

Ebay, limited to within 50 miles or whatever, is lousy. If I were really getting specific, the selection on Ebay would make the shipping fee worthwhile.

OfferUp is a big mess. I think there are MANY on there who post stuff for sale that have just gotten excited about selling stuff online, and post to any and all of those offerup type sites. The 2 times I have inquired about something, the situation has been lousy. Lots of stuff there is over-priced for used.

Finally: we are on "nextdoor." It covers your neighborhood and surrounding areas, only. You have to enter your address to join, so it truly is just genuine local people. People post all kinda stuff - lost dogs, is this snake poisonous, etc., and also stuff for sale. Bikes you might want will not show up that often, but can. The good thing is you know they are not far away. Also, since they are likely getting ad in front of very few interested people, the price may be lower than Craigslist. Or, make a low offer since they probably will not be hearing many inquiries.

Overall, my thing is that you just have to get used to the positives and negatives of Craigslist. Recently, I asked about a bike - including what was the "size" they noted. If the "size" they noted were true, the bike was for someone 7 foot tall. Another guy wanted about twice as much for a 30 year old bike as what it should sell for. I drive out and looked at it (plus did a couple other errands in the area so not total waste of time), and gave him a decent offer. He got insulted. This was 4 months ago - it is still listed - I search for it just out of curiosity and to laugh at the guy. After that waste of time, I figured out he has several listed at any one time - he is just catching the less savvy buyers bike by bike. More power to him, but I blocked his number after telling him the bike would only sell for about half of what he was asking, and his return texts expressing how insulting and ignorant I am. Well, the bike ain't sold yet, is it? --If you can tolerate weeding through stuff like that, Craigslist is best.

That was for a very small road bike for my young son - I was blessed to see, a week later, a very nice bike - old steel, Campy, well-maintained - at a great price. The guy was thrilled to sell it to a kid who obviously was enamored with it at first site, and would be appreciating it.