Hi Steve,

If you think the offer is legit, what you could do is create a second, Buy-It-Now listing for the phone for the amount offered (or agreed upon) and let the buyer know you'll ony make it available for an hour or two for him to purchase (I'd agree upon a convenient time with the buyer first).

Technically you're not supposed to have two different listings for the same item, but if he buys it you can cancel the auction, or if not, take down the BIN listing. That way you preserve the existing bids on the auction in case he doesn't go ahead with it.

It's difficult for anyone else here to judge the legitimacy of the person contacting you - if they've recenty been active in both buying and selling as you've described that's a good sign.  Check recently sold prices on eBay for the same model phone as you're listing in a similar condition - if the price offered is significantly more than what others have sold for, that's a big red flag that the buyer is likely trouble.

It is pretty normal though for an interested buyer to message with an offer even when 'make an offer' is not activated on a listing.  It happens frequently with me (although I never auction anymore, just sell via BIN) and is often just a sign that someone doesn't want to wait for the duration of the auction, or thinks they might get it a little cheaper this way.

Anyway, good luck!