Why did Moody eat the school's sausages?

Question

These two quotes seem to be in conflict - both from GoF:

Quote 1

Moody had told them all during their last Defence Against the Dark Arts lesson that he preferred to prepare his own food and drink at all times, as it was so easy for Dark wizards to poison an unattended cup.

Quote 2

The stranger sat down, shook his mane of dark gray hair out of his face, pulled a plate of sausages toward him, raised it to what was left of his nose, and sniffed it. He then took a small knife out of his pocket, speared a sausage on the end of it, and began to eat.

Why didn't Moody prepare his own food? Why did he eat the school's sausages? Was sniffing the food a means of detecting poison?

Answer 1

He preferred to prepare his own food and drink, but he didn't absolutely insist on it.

It might be somewhat impractical to never eat food and drink prepared by anyone else. For example, in the context of your second quote, he's just arrived at Hogwarts after a long journey. He hasn't carried any food with him to eat for dinner, and it's much easier¹ for him to sit down and eat at the table with everyone else than go to his living quarters and go to the effort of cooking himself food.

If he can't be safe eating Hogwarts food, where can he be safe?

The food served at the Hogwarts feast is prepared by house-elves, not by (potentially Dark) wizards. Corruption among house-elves is probably unheard of. And they probably get their orders ultimately from Dumbledore, who might be one of the very few people Moody actually trusts. Even if he'd never feel safe eating in a pub or in the house of someone he didn't know, Hogwarts is a different matter.

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(As Adamant alluded to, we have to remember that the 'Moody' who ate those sausages wasn't actually Moody. It could simply be that Crouch's cover was slipping slightly, but for the reasons outlined above, I believe it would have been reasonable for even the real Moody to act in this way.)

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¹ _{Not to mention more sociable, but I doubt that's much of a concern for Mad-Eye Moody.}

Answer 2

Here's the full context for that first quote:

Hagrid had his usual enormous tankard in front of him, but Moody was drinking from his hip flask. Madam Rosmerta, the pretty landlady, didn't seem to think much of this; she was looking askance at Moody as she collected glasses from tables around them. Perhaps she thought it was an insult to her mulled mead, but Harry knew better. Moody had told them all during their last Defense Against the Dark Arts lesson that he preferred to prepare his own food and drink at all times, as it was so easy for Dark wizards to poison an unattended cup.

The focus on the flask is the important part. "Moody"'s flask contained Polyjuice Potion, which he had to drink regularly to remain in his form. He had to come up with an excuse for why he kept drinking out of side flask, and Moody's paranoia served as an easy excuse.

Later in the fifth book, we see that Moody does a similar routine to the sausage thing when eating with the Weasleys.

Mad-Eye Moody was sniffing at a chicken-leg with what remained of his nose; evidently he could not detect any trace of poison, because he then tore a strip off it with his teeth.

So the short answer is the first statement is false. Moody does eat food others prepare (albeit suspiciously), but his impostor once said he didn't to keep up a disguise.