If you are in the information technology space and have customers, you have the opportunity to resell SEO. This assumes you are not an SEO yourself, of course. But anyone in the web hosting, web design, server colocation, or related spaces is a candidate. Additionally, many folks in public relations also have customer relationships that are ideal for introducing SEO.
As you consider who you will resell SEO for, it is important that you evaluate the following criteria carefully. There many shady SEO reseller deals out there that are to be avoided. As attractive as it is to increase your revenues by becoming an SEO reseller, there is also the possibility that you could damage your customer relationship. Follow these steps to avoid that outcome.
1) Only resell SEO for a company with direct SEO clients. This may seem obvious, but think it through a bit. If the company you are reselling for only does reseller deals and has no direct customer contact, they are somewhat insulated from the realities of business. This is not good. 
You want to be working with a firm that understands how it is to sell and retain customers. So ask for a client list from any company you are considering working with. If they don't have any names on their list that you recognize, move on to an SEO firm that does.
2) Ask to see their results before signing up. This may also seem obvious, but you only want to resell SEO for a firm that can actually perform. Again, your customer relationship hangs in the balance. As such, you want to bring a team to the table that knows how to make rankings move. Ask about how they do what they do. Ask the SEO reseller firm what their most difficult ranking is. If they tell you some easy 4-word phrase, walk away. If they show you something extremely difficult, you may have found your partner.
3) Ask about their business background. Assuming the firm you are considering passed criteria 1 and 2, now you want to evaluate how it will be to work with them on a long-term basis. To understand this, the best thing you can do is ask about their business training. Is this a bunch of college kids with no business skills or are you working with a professional business outfit with MBA training? Have they worked at and for Fortune 100 clients in the past? How long have they been in the business world? By asking these questions you should be able to establish how reasonable they will be to work with over the long term. You want to build a long term partnership with this firm that will be mutually beneficial.
Conclusions
In summary, there is a great opportunity for businesses in complementary technology spaces to increase revenues through reselling SEO. However, picking the firm to partner with requires doing your homework. Use the three points I've outlined above to determine if a potential firm is one you want to resell SEO for.