How can you buy someone else's trademark?

You cannot buy just the trademark registration in the U.S.; you must by the whole business and goodwill associated with the mark that is registered. Naturally, that business must still be functioning and the mark still in use in U.S. Commerce as well. Anything less, and you are buying just the piece of paper that comprises the registration certificate and therefore little, if anything, should be paid for that.

Register trademark in Vietnam

I suppose that if the business is not operating any longer and a valid US registration exists that you want to acquire or would register yourself (i.e. it recites goods/services that appropriately describes your business), that registration could have some “nuisance value” in terms of being desirable just to avoid a likelihood of confusion refusal by the USPTO in your application. So if you were to “buy” a registration certificate like that for a mark that may not be in use, label the transaction accordingly and pay the appropriate corresponding amount. In other words, don’t buy the “mark” or trademark right that is not affixed to an ongoing business because that could be a transfer of a mark “en grosse” which would invalidate the mark and the registration.

If you don’t follow this, or if you think it is complicated, I would say that it is and you should work with a U.S. trademark attorney on this.