First of all, the name "tilapia" is often used to refer no numerous species of fish, not all of which are Kosher.
Secondly, we generally do not trust a gentile to make this identification; especially when he stands to gain by declaring it a potentially more expensive variety.
Furthermore, even if we were certain that this fish was Kosher, the knife used to cut it was very likely used previously to cut non-Kosher fish.
Generally speaking, Kosher fish should only be purchased with some skin attached to enable the identification of scales unless it is in a sealed container with a reliable Hashgacha. Additionally, it should be insured that the knife was scrubbed well with soap and water prior to cutting the fish. If the fish was already cut, the entire surface should be scraped off and washed well.
An additional issue that has arisen in recent years is that many otherwise Kosher species of fish may be infested with parasites that entail the prohibition against consuming bugs. There are various lists detailing which species from which countries are and are not problematic.