Although most sellers do their very best, sometimes it's simply impossible to provide completely clean eggs. I've incubated some pretty dirty eggs, and had no issues whatsoever, though I do worry about it when they are what I consider to be excessively dirty. Again, though, I've never had an issue, and I've incubated quite a few eggs.
The biggest issue I personally have with my hatching eggs sold is more from mud than from fecal matter. I keep nest boxes very clean, changing the shavings in them regularly, and never allowing the gals to sleep in them. However, when it rains, or when the show is melting - they'll get mud on their feet and sometimes carry it into the nest boxes, and rub against an already laid egg, smudging dirt on it. Not much you can do about that. Without seeing them, it's tough to determine how dirty is TOO dirty, but if you're uncomfortable incubating them, I simply wouldn't. Just because you spend $ on them doens't mean you must incubate them. I've purchased Marans eggs that I felt were below my standard of darkness, and pitched them (as much as I hated to do so, having paid for them).
