I'm wondering if I can get a virus or trojan without knowing it by downloading apps? Since there are apps for ebay and so on I might wonder if they are still safe? Are there any viruses, trojans or malware known?
No, everything in the App Store is looked at by apple, so it's safe. Apple will look carefully at the code from the developer before their app is allowed in the App Store to make sure there aren't any Trojans or other virusses.
If you jailbreak and download other, unsanctioned apps, then it becomes a possibility. Although I haven't heard of any actual malware in the wild, it is a possibility.