Hatching poultry eggs is fun. The eggs vary in price greatly, depending on how popular and how rare the birds are considered to be. A few years ago half a dozen chocolate orpington eggs went for over £100, now you can get them for £10 if you are lucky.
The best time of year to buy is in the spring and summer. Fertility can be a problem at other times of the year. Before now I have bought a dozen eggs in early winter and only three hatch out, whereas I would expect to get 60-80% hatching in spring and summer. Few egg sellers give guarantee about fertility so it is your risk.
When the eggs arrive make sure you let them rest for at least 24 hours. Take them out of any packing they came in and put them in a bowl inside the house. The best hatching I had from eggs that came through the post was when I inadvertantly left them to rest for nearly a week before putting them in the incubator.
Always leave feedback. Good egg sellers like to keep track on how many are hatching. If any eggs are broken in transit then it is worth letting the seller know before leaving bad feedback. I have experienced postmen ringing the door bell to carefully hand me the packet of eggs as well as throwing a package of eggs over the garden gate before letting himself through!