Have you ever gotten a page to rank number one in Google for a competitive term?
And, are you absolutely confident you can get a new page on a new website to rank number one for a competitive term? And are you absolutely confident that you know exactly what you would need to do to accomplish that?
And are you absolutely confident you can take an existing page on a website and modify it and get it to rank number one for a competitive term? And are you absolutely confident that you know exactly what you would need to do to accomplish that?
And do you know how to tell the difference between a good keyword and a bad keyword?
If you answered no to any of those questions or if you are unsure of your answer, then don't start an SEO business. Period. You don't know what you are doing.
That would be the same as just saying "I think I'll be an electrician" when you don't know how electricity works. You are probably going to electrocute yourself or someone else or burn someone's house down because you don't know what you are doing.
If you aren't supremely confident in your SEO skills, but you still want to enter the field somehow, then go ahead and offer link building services of some kind. But sell those services as link building services. Don't sell them as SEO services. The difference is, if you are selling links, then links are all you need to deliver. If you are selling SEO, then rankings are what you need to deliver. And it is a lot, lot harder to deliver rankings in most cases than it is to just deliver links.
What you will probably discover if you try to sell link building services is that the public wants to buy the wrong link building services. If you really know what a good link looks like and what a good link building service looks like, you will discover that it takes immense effort to educate a website owner about what a good link really is and how much that good link is really worth. The vast majority of website owners are so misinformed about this topic that they think they need "x", and they want to buy "x", but what they really need is "Y" and they don't want to buy "Y" because it costs too much because of the effort involved and because they are so misinformed that they don't believe you when you tell them that what they really need is "Y".
Unless you are dealing with real business owners of real businesses, you will probably find that most website owners are dirt cheap when it comes to SEO and often times flat out stupid. You will get people who think that if they buy links from you and point them at their crappy good-for-nothing affiliate site which is part of some ridiculously scammy affiliate program, that they are going to be rolling in the money they make from it. And you can't fix stupid. Sell those idiots links. Don't sell them rankings.