If it is a large chunk [as you said] of your hard earned money, do not shell it out unless you are really sure. In the worst case, you will still have your money with you. In the other case, you lose your hard-earned money forever, plus time and emotional energy. When in doubt, walk away.

Along with the other questions shared by HNers:

What kind of social media management are they doing? Is it in a fast shrinking area? What has been the CATEGORIZED revenue flow FOR EACH SERVICE CATEGORY for the last three months? How about for six months?

What are the revenues generated from each of the top ten clients? Is it skewed? Or, are there only three or four clients?

Is the revenue based on a (monthly) subscription model? Or do you have to keep selling every month?

What will make the clients to stick with you, post-acquisition? Will the seller do something similar after the sale? Does the seller have any involvement with any similar businesses? Any of his friends or family in the same business?

Why are they really selling? They may or may not tell you that, you will need to infer that.