what could the big difference be between a $250 pc with the same specs as a $700 pc?

im a pc technician and can say that they both most likely have false ads is all, a 700$ before covid on a decent pc should get you an (for prebuilds) 8gb ram, 4 core ryzen most likely a 2200g or an intel 4 core system, with like a 400w psu or 500w if lucky, and like a gt 1030 in all honesty, which is decent for upgrading for starters, but for a 700$ pc today, you cannot get what you pay for due to covid, but you can get a laptop with 8gb ram and a 1650 that should be able to do high setting 1080p gaming for sure semi decent until you update the ram, that 250$ has probably been stripped for parts and maybe a 300w psu while the 700$ should have at least a 400w evga

The main difference is that it could be used or the 700 dollar pc may contain some rgb lights, better fans, or a better case. Overall, the 700 dollar pc would be more premium in aesthetics compared the 250 dollar pc.

does it have an i5 or a celeron in it , hard drives could be different or the pc could be refurbished instead of new .

the cheap one could lack quality and come from scary places like Taiwan or china. The longevity of the machine is unassured. 

the cheap one could be used...

I'm not sure as I still own a Gateway 2000.

There could be specs that are missing from the listing or a shady vendor is selling it.  When prices don't make sense you have to question everything in my experience.  

If the components are identical, it's branding. 

WHAT ARE THE SPECS FOR THOSE 2?

The price.