Just taken delivery of three music books for Flute and Piano from three different second hand book dealers. Flute part missing on two of them. NEVER AGAIN.

Do they take returns?

Always happens on Ebay - some 2nd hand dealer once tried to fob me off, saying that only beginner flute music had separate parts. I resisted the urge to ask what one would do if piano and flute were playing at the same time.

Numpties

Contact them through ebay - it may be a genuine mistake that they can amend. If not, then leave them negative feedback. It won't sort your music problem out, but it will alert other buyers.