Found a site you'd like to buy and need help with investigating it?
There's no known service out there advertising due diligence on the smaller web businesses.
The way I see it, you have two options.
Option 1: Start a thread in this section asking for opinions. This forum has probably the highest concentration of experienced website buyers in the world. Some will drop in and possibly tip you off to a flaw or two you hadn't spotted yourself.
Option 2: Find someone who can work with you and investigate a site more thoroughly on your behalf. You can use the list below [Edit: List has been removed. See this post]]. These members have all said they'd be willing to consider taking on private DD work. Browse through their posts, pick the one/s you think sound like they know what they're talking about and drop them a PM. You're basing your choice on what you've judged of their suitability to assist you. You may get some help free, you may have to pay for some help, you may be able to reach a barter agreement. Whatever it is, that's between you and the person you've chosen.
Please do not PM me about due diligence work. Please do not PM to ask me to recommend someone.
[Edit: obsolete content removed]