OK, a lot of personal opinion here but if you're new to the market and looking to buy your first low-cost site, here's what you need to avoid at Flippa (or maybe just avoid Flippa ).
Sites being sold by career sellers / siteflippers
Don't get me wrong, this presents a nice, cheap way to get a complete and working site, but that's all these are - shells - and you have to build a business out of it.
These sellers are long on talk and short on site quality. Avoid them. Click on the seller's name in the listing to see how many sites he's got listed and/or how many he has already sold. If he looks like he does this for a living, the chances are it's a template site which will require a lot more effort than you think.
Zero profit sites and site with < 12 months profit history
It's easy to artificially increase profits for a few weeks/months. I won't go into the details but the longer the profit history the less likely figures have been fiddled.
Sites that have been built on domains that recently expired. These may show PR and show age but you can't rely on those signals. Have a look at archive.org and see what the site used to look like. Did it used to belong to a company or organisation and is now not showing that company's name? Has the look and feel changed substantially? Has the subject matter and content changed? Then it's likely a zombie. The seller is counting on you being stupid enough to not realise the scam.
Sites with large changes made recently
Recently revamped sites, sites which the seller claims he has done a lot of work on recently etc., are suspicious because you don't know what he's covered up. And why would someone put a lot of effort into a low value site if he's looking to sell it?
Sites from sellers who own a lot of other sites
Unless you can work out the connection between this site and his other properties a lot of the current traffic (even SE traffic) could be as a result of the clout wielded by the seller's other properties - influence that he could withdraw at any time
Sites with extensive custom coding
Sellers try to present this as a plus but unless you're willing to get dug into the code to fix bugs ... avoid these.
Quick fire sales
If the seller's in a hurry, claims he has someone else interested in the site, or is otherwise putting pressure on you to take a quick decision ... avoid.
Any I've missed?