A 5* home changes to each horses indiviual needs. For example a nice big posh livery yard where the horses are out in the day in a night would not be ideal for a horse that has to live out 24/7,in the same way a field and a field shelter would not be ideal for a horse that is bred to showjump. Genrally,a 5* home is a home that offers the horse warmth,keeps it clean,feeds it and loves the horse,as well as meeting all the needs of its work.
Its pretty meaningless really, to me it just shows that the seller has some concern about the horse's future and wants a good home. I don't think it means they are expecting miracles, just decent normal conditions and care. But who is going to turn up and say 'oh well, I'm going to be a bad home, but will you sell to me anyway?'
I guess it gives them an easy get-out if some numpty phones up planning to keep the pony in the back garden for the kids to play with.