I want to buy new speakers for 50,000 and see it used for 30,000, what should I do?

My personal experience is this: I have purchased speakers used that I could not afford new. In my case I was happy with the used ones and the 11K I saved- which went into a good DAC. I was happy with the allocation, Additionally the mfg. warranty transferred. No issues arose and I use them to this day. 

I have also purchased new. (times change, wealth changes, etc.) and I was not especially bothered or impressed by "brand new" although the speakers were and still are impressive. 

It's totally an individual thing. To me my AMG 63 was a steal 18 mos. old. I saved 35K. It felt new to me. So did the used speakers. Because I'm a number nerd I consider how much 25K or 35K is worth in a decent investment multiplied by 20 or 30 years, As Warren Buffett says "It soon appears indefensible".  I'm 62. That wealth will get used either by me or my heirs. That's defensible. Meanwhile I have luxury for "relatively cheap". 

Spend the $30,000 used then use the $20,000 to feed A LOT of malnourished kids...

Who doesn't love instant depreciation? Buy new!

For that kind of money, and potential savings, spend the money to visit the seller and get a demo. 

If you want to pay 30k for used then look for used speakers online. 
Email some retailers for demo models 

should be pretty easy to find $30,000 speakers used