You saw the value the 1st ever built Mac brought right? It sold last week. Approx. $220K if memory serves correctly. A non-initial build serial numbered iPhone sealed/not sealed will not bring anywhere near this amount. I'd imagine a sealed, boxed 1st Gen iPhone would bring appx. $700 to $1K in about 5 years or so. I would also argue this would be the max price you'll get for this phone for the next 10 years or so. After which, it might go up a little bit more...but not much more.
If you're not hurting to sell it, hang on to it and see what happens. If you're hoping to cash this phone in on a retirement plan....sorry, but it's not happening! If you're just curious, I can assure you the phone you've mentioned WILL NEVER SELL for more than the actual 1st Mac ever made which just actioned for a bit over $200K!
Cosmo