You are passing the listing_id to the PayPal OAuth endpoint; but where are you trying to access it? If you intend to use it in your callback endpoint, then you are depending on PayPal to forward these query params, which I expect it does not.
Instead, you could use the Session to store the listing_id and inside the callback endpoint (which authenticates the User), redirect according to the listing URL:
session(['listing_id' => $listing_id]);
return Socialite::driver('paypal_sandbox')->redirect();
Route::get('/auth/callback', function () {
$user = Socialite::driver('paypal_sandbox')->user();
Auth::login($user);
$redirectUrl = session()->has('listing_id')
? route('listings.show', session('listing_id'))
: route('dashboard');
return redirect($redirectUrl);
});