You don't necessarily know how long they've had it, or how carefully they've stored it, etc.
eg, surface-mount parts can be susceptible to moisture absorbtion if not correctly stored...

Also, "surplus" could mean anything from "brand new, in unopened, original packaging" to "factory floor sweepings"

The main issue, for a commercial product, is that you just don't know - and the cost (ie, time) of finding out for sure is likely to outwiegh any saving over buying new.

Of course, there may be other mitigating reasons and/or circumstances to justify the choice...