I have seen sites claiming to make $xxx in 7 days or 10 days or $xxxx in 30 days and the site is not even up for 3 months. How is that possible?

Anyone with experience can tell me how they pull this off?

I don't believe they can do that without much advertising.

They're lying their dirty little asses off.

lie or click themselves

I ran into a guy buying niche targeted traffic from webnetjoe and selling quite a bit in his first week. Then throw the site up on flippa, sell it and tell the buyer where the traffic was from but at that point any traffic sent from the same source had either already seen the page or bought already. Maybe a few sales would trickle in but not near enough to cover the cost of the site at that point as he had jacked the price up.

Shady crap!

PPC--> Offer
can be one option ..

If there is no proof of revenue available for these sites, the sellers must be telling a lie then.

Not all flippa members do this thing, their are honest seller as well as buyers. coming to the 'type' of seller you mention in your question, here is my opnion.

usually they also attached the payment and traffic proofs too. but think yourself why they are selling a website for $500-$2000 that is making about $3000-$10,000?.

As I think, they attach fake proof of payments they received
and use auto/bot to generate/show traffic to their website, that do
not add any value to the website.

Calm down guys, PPC, Other sources of traffic can bring in $xxx in less than a week.