I have sold literally 10 horses, and at least 5-7 have been deceased after being brought, and it is by far annoying me a lot. any suggestions how to some how prevent this?!

Unfortunately, that is a risk you take when selling horses. Once sold, the purchaser/owner can do whatever they'd like with the horse. Horses that are unrated, unrated with low-rated pedigrees (ie: sire SSS x dam AAA), low rated but lacking color, etc. and are sold for $1k or less are generally used to free range. The game gives you $1k for free ranging a 2+ y/o horse so some people will make money on horses this way (purchasing low and then free ranging).

Even if you make a deal with someone, there's no guarantee that the foal/horse will be free ranged down the road.