Definitely a good point to raise for the sites with the lower value listings such as Flippa and Side Projectors. You'll find that you can discard the majority of listings on them quite quickly just with your own "looks too good to be true" test. I personally reduce my searches to listings of sites/services that have been operational for two years or longer and have had a long history of traffic and sales.
The other sites like latonas have listings that are generally via brokers and the quality is significantly higher on average though the prices reflect that with multiples of 4-5x and upwards not being uncommon.
In all cases, you'll want to do your due diligence and ask for things like profit & loss statements, read-only access to their analytics, etc. as you're more seriously considering any of the listings.
