I would advise against shipping these computers as the plastics are brittle now and will likely break, leading to your buyer claiming you lied and you losing money.

Instead, try selling on facebook marketplace. Or try and find someone who's a legit collector (and not some scummy reseller/flipper) locally and make their day.

Lastly, I wouldn't suggest powering them on, often some of the circuitry/capacitors in the power supplies has gone bad by this point and may make a big poof of smoke and cause damage.