I think that offering a service to a client that you are not well-versed in is a disservice to the client. Offering, say ... a ZenCart store when you normally do osCommerce is not a big deal. Offering to write their marketing strategy based on a bunch of tactics when you don`t have a solid knowledge of marketing strategy in the first place can be potentially dangerous and damaging.
I don`t mean to pick on you - I just see this a lot in web design and it`s a huge pet peeve to me. A bad marketing campaign is worse than no marketing campaign, so if you`re not sure you should be educating yourself or teaming up with someone else do you can learn.
There is a wealth of information in books, online, etc about marketing, marketing strategy, and marketing tactics. I`m not talking about the internet marketer`s version of writing articles about spamming forums ... I`m talking about true marketing strategy, tried and true methods that have been translated into the web.
Start with some of the great people on this list:Viral Garden`s Top 25 Marketing Blogs
A lot of them have great info on their sites and/or write some really good books on it.