A seller can say anything, that doesn't make it true...
In cases like this, the issue could be anything from a dead spark plug to a destroyed engine; unless you can pinpoint the issue in a matter of minutes, you need to take for granted that the engine is toast...
If you can verify that the engine has spark & compression, those are good signs & narrows the issue to fuel delivery; if there is no spark or compression, at that price, walk away or beat him right down on the price...