'T is pleasant purchasing our fellow-creatures;
And all are to be sold, ...
Based on these lines, I think Byron is talking here about the variety of things that can be used to metaphorically "buy" people. I guess it is a matter of opinion whether the image of humanity as all being metaphorical slaves of some sort is particularly accurate, and it could be considered to devalue the unique harms of actual, literal slavery, but it seems to exist in other works of literature also. Compare these quotes from "Moral letters to Lucilius/Letter 47" by Seneca, accessed via Wikisource:
"Slaves!" No, they are our fellow-slaves, if one reflects that Fortune has equal rights over slaves and free men alike.
Show me a man who is not a slave; one is a slave to lust, another to greed, another to ambition, and all men are slaves to fear.
...some by features
Are bought up, others by a warlike leader,
Some by a place -- as tend their years or natures;
The most by ready cash -- but all have prices,
These lines seem to be listing different kinds of things by which people's allegiances can be "bought" or controlled. For some people, "features" are enough to buy them (I'm guessing this refers to pleasant features of someone else's face or body?), for other people, "a warlike leader" is enough to "buy" their support; for other people, "a place" is what will "buy" them; and others will be "bought" by cash (i.e. bribes). This interpretation seems to me to be supported by a footnote I found by Coxe in The works of lord Byron, with notes by T. Moore [and others] (1842) that relates this passage in Byron to Robert Walpole, a famous politician, who is supposed to have said "All those men have their price" (p 655).
From crowns to kicks, according to their vices.
I would read this as describing the wide range of "prices" for different people: some people are "bought" by things that are typically thought of as good, while others are "bought" by things that are typically thought of as bad.
"Crowns" here I assume refers to the coins. I think "kicks" is just a metaphor for bad treatment, which might actually cause some people to be more obedient like a stereotypical dog that is kicked but still loves its master. (It seems possible to me that it could even be a subtle allusion to masochism.)
I found a source that describes this as a "double pun", but I'm not sure how to determine which of the potential interpretations Byron intended, or if he really intended all of them:
39: From Crowns to kicks: a double pun, crowns being either regal headgear or five-shilling coins, and kicks being either blows with the foot or sixpences.
(DON JUAN CANTO FIFTH, edited by Peter Cochran, p 13; from Peter Cochran's Website)
The information that "kick" was a slang term for sixpence was new to me, and certainly seems like it might be relevant.