Based on my own farily extensive listening experiences with the 1B vs. 1C, I'd say the differences are subtle at best. Most of the changes in Vandersteen speakers, regardless of the model number, involve incremental improvements, and as such you will not find really major differences in the evolution of a given model.
If I were buying used speakers, I'd be inclined to get the 1B's if they were substantially cheaper than 1C's. If your budget will stretch to $500-600, however, I'd suggest you look for a pair of pampered, low-mileage Vandy 2Ci's. They offer a lot more performance than the 1B's or 1C's. The newer 2Ce's have somewhat better performance than the 2Ci's (which have been out of production for about 10 years), but they tend to sell for quite a bit more than the 2Ci's.
If I were buying used speakers, I'd be inclined to get the 1B's if they were substantially cheaper than 1C's. If your budget will stretch to $500-600, however, I'd suggest you look for a pair of pampered, low-mileage Vandy 2Ci's. They offer a lot more performance than the 1B's or 1C's. The newer 2Ce's have somewhat better performance than the 2Ci's (which have been out of production for about 10 years), but they tend to sell for quite a bit more than the 2Ci's.