Stormtroopers kill Owen and Beru Lars, and then incinerate them to simulate an attack by Sandpeople. Moreover, they bring banthas to leave incriminatory tracks (which do not fool good ol' Ben).
But this kind of convoluted cover-up is not something one comes up with on the spot. Recently detached troops would not have enough local knowledge to concoct it, and local troops would not use tricks that moisture farmers were sure to see through.
What gives?
Keep in mind, that they were not just regular stormtroopers. They were SandTroopers. According to this Wookieepedia's explanation on Sandtroopers
They were nearly identical to standard stormtroopers except they had additional training in specialized areas and advanced cooling systems in both their helmets and suits that offered them protection from the relentless heat.
Shouldn't this additional training contain tactics of attack methods, concerning the planets with "sand environment"?
The man who ordered the stormtroopers there was raised on Tatooine. Aside from Lord Vader, who had a very personal familiarity with the Sand People, the simple fact is that the stormtroopers were a highly-trained, elite military unit attached to a very important mission. I'd be more surprised if they didn't do enough research to pull off that sort of deception. And as Tritium21 points out in his answer, some of the stormtroopers may have been from Tatooine originally; they were riding dewbacks and banthas, after all, which indicates that there was at least a small garrison on Tatooine.
As Ben Kenobi points out, the only mistakes the stormtroopers made were related to them being too good at killing to be the nomadic Tusken Raiders. They travelled abreast rather than single file, as travelling single file in the desert would be very poor practice for any sort of combat mission (aside from a stealth mission, which this wasn't), and their weapons fire was too accurate to be from an amateur. That sort of attention to detail is something only another trained soldier - as General Kenobi was - would be likely to notice; to the average Tatooinian, this would be evidence of a Sand People attack. Luke Skywalker, in fact, does mistake the attack as a Sand People raid. The same sort of elite troops that could fire that accurately and track down the Lars so quickly and expertly, would have little trouble in mounting a relatively simple false-flag operation.
To address "and local troops would not use tricks that moisture farmers were sure to see through", you are wrong on both counts:
Recently detached troops would not have enough local knowledge to concoct it
According to this SFF answer, there were local troops garrisoned on Tatooine. Even if they didn't participate in the search, they would have been able to provide intelligence.
Also, as another answer noted, Vader himself was a Tatooine native.
And they had native guides. We see that both in G-canon (an informant telling the Imperials about the meeting in the Cantina; and Boba Fett working for Vader), and in EU Legends canon, where in Tatooine Ghost we see Imperials working closely with the natives to collect intelligence.
These troops did not fail to fool "moisture farmers". They failed to fool Obi-Wan Kenobi.
Leaving aside other answer's points about him being a Jedi, from Miller's Kenobi EU book, we find that he not only lived half his life on Tatooine watching over Luke; but that he was intimately familiar with Sand People and their ways, having interacted and fought with them.
It's even implied in Miller's book that, more likely than not, Ben lived among them at some point. (Although the Legend-ization of EU and Darth Lucas likely prevented a sequel covering that from ever being written.) At the end of the book, the Sand People's leader is convinced that Ben is meant to be the magical savior of Tuskens, just like Sharad Hett, a Jedi who lived as one of them some time before the ANH events.
It is entirely possible that there was a Tatooine native, or a member of the local barracks in the party that attacked Owen and Beru Lars. I have no extended universe knowledge to confirm or deny this, but it logically follows that the force on the ground would be at least partly locally sourced.
He is a Jedi. It is not out of the realm of the force for him to sense who caused what tracks. It's also possible that he is simply more observant about tactical matters (again, hes a Jedi Knight) than a moisture farmer.
