Just make all your horses model horses! And find a breed coin and sell for 80000

I agree with Elowyn, that entering horses in careers is a good way to earn income. It may not seem like much but it definitely adds up over time! Breeding (and selling the offspring), in my experience, isnt a great way to earn steady income. Selling wild horses can earn decent money, especially with rare patterns such as skewed roans. Discipline and halter shows also earn decent money if you have okay-good horses, but I mainly recommend careers. 

If you're at level 5, and the progress bar is almost full, you finished all the quests we have so far :)

The problem is, trying to find wilds/foals that people will want. I just tried selling a training boost 10 Chinco filly foal, twice, for 25k as a starting bid/reserve and she never sold, so I'm keeping her, whereas I've seen other Chincos going for decent bits and or ingots. The market in general, I feel is oversaturated and trying to sell anything is gonna be hard. It also depends on how much you are asking for your wilds and or foals.

Best bet with breeding, is to wait until the stallion and mare are both level 50 and get a foal that will have a training boost of 10, so they can do better in competitions. It does take work but it can be done. Probably best bet is to either continue putting them off to Careers or putting them into Halter shows to get some levels out of them. The training boost of 10 does immensely help as they will be able to gain stats better and do better in competitions to rake in more bits. You could also, tenatively, put some horses into competitions and see if they do better there than Careers and or Halter shows.

As for the Marketplace, do you leave it open to offers, or do you do a flat price? I find anything, if I do sell it, goes better for a flat price. Currently there is fall dyes and you may, more than likely, get lucky selling those for a flat price and can find them via Exploring Hoofbeat Plains or browsing the website in general. While Elowyn does have a point, Breed Coins have become ruined and are waaaaaaaaaaaaaaaaaaay too common to find nowadays and 80k might not get you anywheres as I have seen 40k as the cheapest of them going for as of late. It boils down to, why buy something for 80k when I have seen for half that price? If you even have some items to craft treats for wild horses, I've seen people selling those for flat prices as well. This suggestion is up to you to decide overall if you wish to follow through on.

Quests? If you are at level 5 and 550/600, you are, like everyone else, caught up. According the most newest news post Curly did, Novemeber seems the ideal for new quests to come about.

Dusty is right. As of right now, in the beginning you have a lot of low PS horses. Your focus should not to be breeding and selling them yet. The best bet when starting out is not to buy a lot of horses. Get a small handful of decent ones. Train them 5 times a week and figure out if they are best suited for halter, careers or disciplines and always be entering them. This is gonna be your main source of income, even if it isnt a lot at the beginning.

As Dusty said, wait until they are at least level 50 to breed. If for some reason they are 20 and not at least 50 yet, I would at least breed them before they retire to start the next gen but they should be able to get to 50. You want that 10 training boost. Then work on those foals doing the same thing. Slowly you start to build better horses that are worth selling and breeding to. Its not supposed to be a fast game though.

That doesnt also mean a guaranteed sale either. As Dusty said, she couldnt sell a 10 TB Chinco and they are super popular right now. For some reason, it still wasnt something someone wanted. But when you do have good horses ready to offer, you can try auctions to let people bid on them. Advertising your sales in the forums, discord and shouts can help get the word out too.

As far as selling things in the marketplace, I also agree with Dusty that I have had the best luck with flat-rate prices. When I am selling at a flat buy now option, everything always sells, because people can easily get what they want when they want it without having to wait and see if anyone is ever going to accept their offer. I almost never take offers and rarely bother with any sales where I have to make offers. Usually too many offers are just silly anyways. If you arent sure what to price something, you can ask to get some opinions.

Breed coins were supposedly rare but they really are a dime a dozen now. I personally wouldn't buy one as they are so common to come across in explore now and I have more than I need. Dyes, treats and gems are usually pretty well sought after in the marketplace.