I hope this is not out of order and is taken as the advice to new people that it is. I have no qualifications as an appraiser of bikes, except for being a bit of a cyclo-dork, but do know a few things about prices and buying in specialized hobbies and online forums. I'll try to not duplicate anything that's already in the sticky faq.
1 - The most knowledgeable people on here do this as a business. They know very well how and where to find excellent deals on bikes. They do indeed love this stuff, but are pretty hard-nosed about pricing. IMHO, much of the pricing advice is based on that expertise. When I was buying/selling/brokering tube hifi gear and guitar amps, I'd score stuff for prices that no normal person could match. I think the same principle applies here. I can be two blocks away from a yard sale and recognize a guitar case that deserves a closer look. "Singing furniture" and receivers and amps can be diagnosed from two houses away. With less knowledge of where to look, you can expect to pay more than they do for bikes and more than I did for gear.
2 - Someone who is flipping is concerned about sizing issues and availability of a particular model only to the extent that it affects the price and speed of sale. The normal guy who owns a bike or two, who wants one more is all about HIS size and THE model or type of bike he wants. Lately this seems to be recognized more. But there have been unfortunate conversations. "Large frame sizes are worth less than the usual sizes" may be true from a retailer's POV but not to the guy who's over 6'2 and hasn't seen anything that fits.
3 - Advising patience is always in order. The law of eBay and CL and flea markets is that the best deals go to those who wait. If someone here tells you that something isn't a good price, just read it as that, advice to wait for a better deal. You aren't being called a fool for considering it, just that prudence is in order.
None of this is to criticize the guys who take the time to offer their expertise. It's great that these cats help people without consideration of their own financial interests. Thank them and consider the advice seriously and gauge it to your own priorities. One aspect of that is skill. If you can do your own bike work and have the tools, the range of bikes that make sense for you expands. If you go to the shop to get your brakes adjusted this market may not suit you at all. If you can pull and service a bottom bracket you can find deals that others must pass. Nobody here can know exactly what your abilities are, so consider that with the advice.