How important is the manufacture warranty? Does a price difference of 250$ is reasonable, if I get the wheels with the manufacture warranty (as if I had bought them from an official supplier of Zipp wheels)?
I’m definitely NOT a wheel expert - but did recently buy a set of Zipp’s (2008 404 + 808). Sounds like you are asking if $250 for some kind of warranty on these used wheels is a good deal? If so, then depends on the asking price + $250 compared to new wheels; I’d go with the ~60% of new figure by a previous poster for any wheel but zipp, they seem to hold their value pretty well if you look at sales on Ebay, more like 75-80% for newer models. I bought new because it’s probably a once-in-a-lifetime purchase, so for me spending a few hundred extra dollars was not an issue. I keep my gear a looonnnnggg time.
I’d ask myself how many miles on the wheels, how heavy was the rider compared to me, what kind of roads, or only used in races? Zero spoke tension, is this even possible? You should easily feel this, or hear it if you do the ping test by tapping with a spoke wrench. A loose spoke will sound flat, all spokes on one side of rear wheel should sound the same, all spokes on both sides of front wheel should sound the same.
Most Zipp owners swear they are pretty rugged for race wheels, and in fact Zipp makes a case for durability being a very important design criteria, unless these wheels have suspicious history, a wheel with low race-only mileage is probably pretty safe, IMHO.