[mothered]

I'm gonna plan to SE some electronics the next week and want to do it with PayPal dispute
I saw many many SE'ing services are offering this

They're most likely using the flawed FTID or LIT method, whereby PayPal only requires the tracking number to verify the consignment Is "being delivered" (and not yet received) and Issue a refund thereafter.

And what reason should I give PayPal for a Dispute?

In PayPal's terms, "SNAD" (Significantly Not As Described) which In social engineering parlance, Is the wrong Item received method.

As such, you'd (seemingly) return It as per my comments above.