It's hard to say for sure and there are a number of factors that can affect what you see. Some may be real numbers, others may be things like robots or spiders that hit the domain and "click" on every link. I have about 500 domains and for the past month now my income has hit as high as $200 a day, only to be adjusted back to as low as 33 cents. Along with the real traffic I seem to get a lot of automated traffic that I did not cause.
Anytime you have a seller with good traffic you are going to want to ask a number of hard questions. It's easy to go out and buy raw traffic that shows good visits if few clicks, and not that hard to get someone or something to generate the clicks as well (Circle clicks, Pay-To-Surf, etc.).
The main thing to look for is a new user, only a few posts, or low trader rating. Then if you see things like 1 click, 100% CTR for a number of days (Saw one domain posted just today) it should raise a red flag. It is possible to get some high CTR, but if you look at the traffic pattern over time you may see things that make you think something is not right. Still, from looking at my own domains I see strange things...
Most services will also provide information on where the traffic is coming from and what phrases were used in search on the domain. If you can get access to that information also it may help you figure out if the traffic is valid, so ask the seller.
Also, I have not heard of anyone doing this, but I see no reason why you could not contact someone at the place where the domain is parked. Explain that you are a potential buyer and while you understand that they cannot provide any information about the person's account, you would like to verify that the parking service is monitoring the domain in question and that they are satisfied the traffic is valid. This make a lot of sense now that I think about it, because if you buy a revenue domain and move it to another service it may not perform as well as it is doing now. If you can, you may want to try and have the domain set up under your account with the parking provider and retain all of the same settings.
Along this line of thinking, asking the seller who their account manager is at the parking service may tell you right away if they are running a scam.
Side Thought: Perhaps parking providers can also be asked to provide a "push" option for domainers?
Another valid option for buyers is to ask the seller to allow you to park the domain under your account for a short period of time. You may have to agree to pay them the equivalent amount of money that they say the domain is getting per day, but if you are not seeing the traffic or getting the revenue you expect, you can tell them to stop right away and avoid a purchase mistake. For some parking companies you may have to notify them you are doing this. I cannot park new domains in my main account unless my information is on the Whois record.
