Buying secondhand flutes is much like buying a secondhand car. It all about how much use the flute has had not how old it is.
A 20 year old flute might have had no more than 10 hours use.A 5 year old flute could be worn out.
The most important aspect is that the seller is a player and can describe the condition and playabiity truthfully.

Feedbacks will indicate the honesty and efhics of the seller.

Always buy respected makers.  Yamaha, Jupiter, Newer Buffets, Trevor James,  Emerson. Avoid all chinese flutes and older flutes than 10 years because they usually play out of tune.

flutes under £100.00 will usually be  trouble so go for a good maker and pay a bit more.

I am a teacher and a proffesional player and I have had some amazing buys on ebay but some poorly
described instruments also. ebaying is a bit of fun and you must play by the rules but not get  to serious.
It much better than Traipsing around shops ,thats for sure. Good ebaying.