have him show you the horses eating frozen mysis shrimp. If they eat it with gusto, then you have mAde the biggest hurdle to keeping horses.
Wild caught tend to have parasites. Ask them were they got the horses and if they are wild caught.
Also ask what kind they are. Different horses have different water temp needs. You will need to turn down your tank temp to a maximum of 76 for most of the "EASIER" species, lower would be better. This means you will need to keep them in an air-conditioned room.
You can keep them with other animals and corals, but your choices must be careful or you must make tank design changes. I keep 4 Kuda in a reef tank, but the tank is is essentiallly two tanks because I have created a rock divider that splits off one third of the tank for nothing but the horses. They occassionally find their way over to the other side of the tank but usually only when they are hiding because they want to hatch babies, or are looking for some pods to snack on between meals.
In then end, I paid 250$ for 4 Kuda seahorses via the mail. There were captive bred and already eating frozen mysis. I use Hikari brand because it is smaller. Hesitant as I am to suggest it, if these 20$ horses are eating frozen mysis, and your 36g is set up correctly and cycled, then I would say go ahead and get two. Try to get a male and female if you wAnt to try and raise babies, or two females if you don't. You be risking only a small amount on with two of them.
Yes... we all want you to get captive bred for sure, but sometimes there are opportunities to not pass up.
I caution with these notes:
1) make certain they are eating frozen mysis
2) make certain your tank is ready for them
3) only get two, that way you loose little if they die
4)*** don't mix species because of disease issues
5)*** don't put CB horses in later if these are not CB unless you are ready to infect new horses with whatever ailments the first pair has.
6)*** if you keep anything else in the tank, don't keep anything that will compete for the mysis food of your horses. Other animals will eat the mysis, but you'd want animals that don't eat lots of it too fast for the horses to eat their share.
Good luck, Kevin