Do I assume all of my eggs are fertile?

Not 100% but that is a good hen/cock ratio. How old are they? If established layers and mature cock then most should be fertile

 

No. Have you observed he rooster mating with all the females? Even then there is no guarantee that the hens will be laying fertilized eggs. I have read that after mating the hens will produce fertilized eggs for five day however. In any event you would have a 50/50 chance of having fertilized eggs. My roosters mate with all my hens and I though I would try incubating some. After a couple of weeks I candled the eggs and there was no development in any of them. So, you are taking your chance and it may pay off with chicks. Just don't be too disappointed if it does not.

 

That really depends on the seller - some will sell them as hatching eggs just by virtue of having a rooster in the flock, others will sell them out of "breeding pens" where they have a particular number of hens to roo ratio to give the best chance that he is covering the females reliably rather than a flock with so many hens that chances are not everyone is receiving attention on a regular basis/he is spread a bit too thin, still others will even go so far as to have cracked test eggs periodically to see if those eggs are fertile which would indicate other eggs from that pen/flock are also likely to be fertile or will sell them out of pens/flocks that they have successfully incubated the eggs from.

 

I would never assume that any egg, no matter where from or what guarantee, had more than a 50/50 chance of being fertile. Even Murray McMurray says "Because of conditions beyond our control during incubation, we are unable to guarantee hatchability. However, all the eggs we ship will be fresh and guaranteed to get to you in good condition." They seem to indicate that there eggs are all fertile but your method of incubation might mess thing up and nothing would hatch. So, no guarantee of fertility.

 

A seller might guarantee fertility, and condition upon receipt because of packaging prowess, but they wouldn't(or shouldn't) guarantee hatchability....too many variables.

I don't sell hatching eggs but check almost every egg I open to eat just out of curiosity and because I've done some hatching myself.
In case you don't know....how to tell if an egg is fertile.