でよければ is an expression that is constructed with the 格助詞（かくじょし） (case marking particle) で, and the potential form of the い-Adjective いい, よければ. The literal meaning of ‘if with (A) is good/fine’, or ‘if by (A) is good/fine’ is quite similar to the standard translation, which is ‘if (A) is ok’.
でよければ will always follow nouns or noun-phrases.

• 焼（や）き肉（にく）でよければ、うちの近（ちか）くに美味（おい）しいところがあるから、そこに行（い）かない？
  If yakiniku is fine, there is a good yakiniku place near my house, so do you want to go there?
• 中古（ちゅうこ）で良（よ）ければいいのが沢山（たくさん）ありますよ。
  If used ones are fine, we have many good options.

でよければ is a phrase that is often used in relation to oneself, in order to politely offer some sort of help, advice, or service.

• 私（わたし）でよければ手伝（てつだう）うよ。
  If you don't mind, I can help you. (If I am a suitable option, I can help you)
• 先生（せんせい）でよければ、先生（せんせい）が写真（しゃしん）を取（と）ってあげるよ。
  If you don't mind, I can take a picture for you.

As で is being used in its standard meaning of ‘with’ or ‘by’, でよければ implies that the speaker is enquiring whether the quality/ability of (A) is good enough to achieve some specific result.