Is $40 used a good price for an Ibanez ts9?

Hell. Yes. I'll explain later. You have a pedal to buy...

 

40 bucks is a good price for any pedal if you like it.

 

Yes.

 

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https://reverb.com/price-guide/guide/2504-ibanez-ts9-tube-screamer

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Reasons:
1- If you like the Tube Screamer sound, this is a damn cheap way to score yourself an original. It's not the "classic" TS-808, but it's three components away from an exact conversion, and some people prefer the TS-9 sound anyway.
2- That's pennies on the dollar compared to what any number of boutique Tube Screamer clones goes for nowadays. I could build you one myself for less than $100 if I'm feeling generous, but $40? You could buy the box to put it in...
3- It's 40 bucks for an original TS-9! That should be reason enough!

 

Seems like a fair price. I paid $50 for a craigslist TS9. The only caution I have is that this pedal is popular with DIY modification, and if it has been passed around the used market, then there's no telling what has been done to it by previous owners...and the current owner might not even know.