Can you please give me a ballpark value of a one-owner all-original Gibson ES350 in original case, never repaired, in near mint condition?
While I don't provide specific guitar estimates, there are many resources online to help figure out this information for yourself. One of easiest ways to go about estimating the value of a guitar is looking at similar guitar auctions on Ebay.
For more guitar value insight, Blue Book Publications offers the Blue Book of Electric Guitars (as well as the Blue Book of Acoustic Guitars, and the Blue Book of Guitar Amplifiers). Users can order the book, order the information on CD-rom, and even get access to the information immediately via online subscription (from $29.95 per year).
For those serious about getting their guitar appraised, the highly respected George Gruhn (of Gruhn Guitars) offers online appraisals for $35 USD.