Regarding the buff missing error on the Currier and Ives "The Road-Winter" (sleigh) stamp, I originally wasn't going to chime in on this, but here it goes...
In the "sleigh" stamp, the buff color missing error as a USED single is exceptionally difficult to identify/certify. There are a lot of reasons for that, which I don't want to get into. Scott catalog users will notice that Scott doesn't even list a dash for the used stamp in their catalog. While not impossible, it is difficult to get a certificate for this error in used condition, and likely not worth the expense.
The buff color on this stamp is a very light shade that is almost transparent. Coupled with color registration issues, color variation/changling issues, it's tough to ID on even a mint stamp. This is why many error collectors prefer to collect this stamp as a plate block or full mint sheet, so they can "see" the "missing plate number". About 75-100 of these sheets have been found, and some have been broken apart to sell the plate block, so there are mint singles available.
While it's fun to "discover" an error in your collection, I'm nowhere close to being good enough to spot this buff missing error as a single. If you guys/gals can do it, congratulations, and I'll definitely be impressed!

Again, the pointers given by t360 and nr-notrare are useful and accurate.
As for me, this error is so inexpensive that I feel it is easier to buy it from a dealer who is tearing them off of a partial sheet of known errors. I normally prefer the "discovery" route myself, but this is really one of those stamps, that if I wanted it, is not worth my personal uncertainty/frustration (just my opinion, sorry, if I discouraged anyone). These singles torn from partial sheets typically sell for $5-$7 from very reputable dealers, which is less than the cost of certification.
Meanwhile, have fun trying to figure out if it is indeed the buff missing error, and I'll be interested in reading your final conclusions on the stamp! Good luck!

k