hi is it okay to mix 50 ohm and 75 ohm cables for a listen-only radio, any help would be appreciated.

The fact you are asking suggests its a long run?
I'd suggest if on a budget get a roll of satellite co-ax, its cheap as chips and I've used it for receive no problem.
Better than having joins and mixed cables.

Technically = NO

In reality, try it and see if you get acceptable results...

yes, I may try it and see, if not happy will get some satellite coax thanks for your help.

Assuming you mean HF, would be problematic at VHF/UHF
Used 'what's in the junk box' in the past for lower frequency receive, not noticed a problem.

Of course you can mix them you even even transmit using them, I do it everytime I transmit using a 1/4 wavelength 75 ohm onto 50 ohm coax.

You still have not said what "frequencies" you want to "just receive".
If its for HF/general low frequency broadcast bands with a general wide-ish band dipole or offset something or other it's not that critical, and you could get a decent run of 50 ohm RG58 style coax that would do the job for not much outlay.
If you are a VHF/UHF/ (and above) "radio scanning enthusiast", you might want to consider the best high frequency capbale/low loss cable you can afford, to make the best out of whatever "up in those regions of the spectrum wideband antenna" you have in mind for your "general receive only reception" hopes and desires.
Just saying.

Personally I would mix/match impedances and bodge with couplers, as every connection you make adds a bit more loss, and far better a straight correct impedance run from terminated point A to terminated point B.

You mean WOULD NOT ????

Indeed the word "not" was unfortunately omitted, it must be the the old age Dementia kicking in!
Her indoors had to remind me it was my birthday on Sunday just gone, as it had completely gone out of my mind.
But seriously I have seen this argument and questioning before...namely "It's only for receive... so will anything will do?", but in either direction, be that transmit or receive, a suitable correct impedance match and as efficient a low loss RF signal path as you can get from whatever employed antenna always pays dividends!
A cobble up, always behaves and performs exactly like a cobble up.