When buying SEO services you need to be careful because using services from the wrong provider can harm or even kill your website or blog.
Make sure to shop around and compare the services they offer and what exactly you are getting for your money. Try to find one with some kind of guarantee.
Find out exactly what they will do (They won't tell everything but at least a general idea is good) for your site and what you can expect.
Try to use a service that has a good reputation, don't just go by testimonials you see on their site. Try to do a Google, Bing, whatever search on the company/person who you are thinking about using and see what information, good or bad there is that you can find.
The thing is you need to be reasonably sure that they aren't going to use any black hat SEO techniques that might get you a rankings boost now, but a penalty later.
I would just shop around and don't jump into the first thing that comes along without doing research.
Good luck and I hope this helps.
