Does going to Jeddah to buy gifts during Hajj invalidate the Hajj, and what are the consequences of that?

Your Hajj is valid as long as you have fulfilled its pillars and obligations and have not committed any prohibitions. Your going to Jeddah after the first tahallul (partial release from ihram restrictions) and then returning to spend the night in Mina entails nothing. What is important is that you spent the nights of the eleventh and twelfth (of Dhul-Hijjah) in Mina and stoned all three jamarat each day.