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It's too good to be true.
Literal
True to-be [for-には] too-good.
A more literary/formal rendering of 'too good to be true.' である is the written/formal copula, used instead of だ in elevated or documentary-style prose. 本当であるには ('for it to be true') uses には as a purpose/criterion marker. ~すぎる attaches to adjective and verb stems to mean 'too X' — here よすぎる ('too good,' from よい + すぎる). More bookish than the colloquial 本当にしては良すぎる.