for "Thank You", I think gratias tibi (vobis for plural) ago is more of a standard (assuming first person).
If someone asked "Quid agis?" and you wanted to reply you were not well, I suppose you would use an adverb like misere or male, a verb as solitario suggested, or a statement such as non bene est or miserrimus sum.
Ave means "hail" and can be hello or good-bye. Saluto means "to say hello, to greet". You could say eam salutat and it would mean "he greets/says hello to her", but this could never just mean "hello".
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae