Is it worth it? Only you can answer that question. If you can't get the seller to remedy his mistake, just get the bike and ride it and see what you think.
Looks like you're in Annapolis... I have a compact crankset (50-34) with an 11-25 cassette and I get around fine here. 39/25 might be a little higher than I'd like for some of the worst hills (which I don't think there are many of honestly - the only thing I can think of is climbing the Naval Academy bridge, which is at least short) but I could probably manage. Some of the stuff up near Baltimore like Lawyer's Hill in Elkridge I need a wider cassette for.
A new cassette would run you 100 - 120 or so, and changing the crank to a compact would be 150 for a new R600, more if you want Dura Ace-level stuff or if you have to pay someone to install it.