Would the possessive of Yahoo! (the search engine) be Yahoo!'s? The exclamation point is obviously a part of their trademark name. For example: Yahoo!'s stock has increased considerably. And if Yahoo! ends a sentence, do we need a period after it? For example: He looked up the information on Yahoo!. Thanks.
In the first case, "Yahoo!'s" is correct. I'm not sure about the second case, though a lot of magazines and newspapers actually drop the exclamation mark. So for both cases, you could use "Yahoo" instead of "Yahoo!" and it'd be fine as long as you're consistent. Also, Yahoo! is technically Yahoo! Inc. So if you're going to be formal about it, might as well write the full name.
you can also word the sentence so a possessive isn't needed and the brand name isn't the last word... that would be the simplest and wisest choice, imo... and the brand name can also be enclosed in " "... so it could read like this:
