Perhaps the external company thinks they need more links. Though were my site a PR7 I am not so sure I would still be looking to buy links.

Perhaps they have been recently contracted and think that this is what the previous promoters did.

I would imagine this company has been hired to target various new keywords/phrases.

I have been approached about selling links by a couple of HUGE websites recently, but clearly these guys have the budget to fund these campaigns.

Yep they offered me pretty good money for the link but we've never gone down that path so I declined.

Link buying is frowned upon so why take the risk when your site is strong?

It's generally the link seller that is taking the risk!

"link buying is frowned upon"

By who? How do you know which links are paid and which are not?

Google!

Surely they (search engines) can only know it's a paid link if webmasters admit its a paid link, No?
Or do crawlers have an algorithm that can somehow tell it's a paid link?

There is a lot of risk involved when you are considering buying links although I would say they could be undetectable by Google if it is a "good paid link". These are painstakingly done to avoid detection like deals done via emails or personal meetings and is not obvious that can be detected by Google's algorithm. That's why Google has a better way of detecting them -by getting people to report such paid links activities. Still, you may have to weigh such risks before you decide.

Google!

I guess it all depends on what you would call buying. Is doing a favor for someone else and getting a link classified as buying a link?

I don't see how linking out from good content, for example, to said pr7 website is any different than a link out of text from a popular blog, news article or .gov content piece - regardless of whether that link is paid or not.

maybe there are some risk

I cannot see a problem with paying someone a little bit of pin money for a link on their relevant blog.
This is very hard to detect, and if you don't state the 'bleedin obvious' (monty python) you should be fine.

Bek.

It's generally the link seller that is taking the risk!

Only google can make money on paid links, don't mess with the beast.

Not true its not only Google that make money on paid links, look around you. Mind you it's difficult to tell who's paying and who isn't.

;)