I've gather eggs/straws from horses that are about to retire or I'm going to sell if I want to keep those lines. It is pretty expensive so I'd suggest only getting it from ones that could sell if you don't use them. I only pull from my best studs and mares. 

I think the best horses to pull from are the one's who are older, VERY well leveled up, and have really great stats/color/other measure you breed for PLUS very few offspring. I personally try to avoid inbreeding (mistakes happen 😬) so if I pull straws from a stallion I've already bred a bunch of times then my options for future breedings are pretty slim. Pulling straws/eggs is expensive and there's no guarantee you will get a foal in the future so make sure the horse you're pulling from is worth it! 

As for selling, you can sell straws/eggs in the marketplace but there's not a huge return on investment in my experience. Unless you have 5* 6E special coloring/genetics you will probably have a hard time getting your money back, especially since regular "live cover" breedings in game are at or less than the cost to pull straws. So I only pull from horses I know I'll want to breed to in the future so I don't have too much left that I then have to sell or destroy.

I pull straws and eggs from my old horses only and usually only the ones that have color. The timeline for breeding for color and high stats is really tight. So I'll pull when the horse is 18 yo + to make sure I have enough chances for the color to gene to also pass.

Personal advice, only use the $25k or the ingot option. The cheap options will cost you more money over time since the failed breeding rate is high.

Thanks guys! I decided to work with straws and eggs, there is some special horses that i want still be around. Thank you!

Can someone help me with how to get straws and eggs, and how to use them? I bought q straw from a trade and now I have no idea what to do 😬 also I'm about to retire ome of my best horses and would love to save some from them!

You have to go to the Lab (which can be found in Town) and from there to the Breeding Clinic. In the Breeding Clinic you can collect straws and eggs from your own horses, make an embryo from a straw and egg, or implant an embryo or straw into a mare. Make sure the horse you're going to take a straw/egg from or implant a straw/embryo into has enough energy, since these procedures take the same amount of energy as a breeding.

And how we sell them? The Eggs and Straws?

They're items, so you would add them to trades.