Roni, my senegal, is only a year old. She was hand-fed, but weaned when I first met her, and she was not mean at all but also was not used to having people handle her by the time I started. She was in her own environment for a long time before I moved her, so I had some advantages you will not have. I was able to move very slowly. The first time the only thing I did was have her sit on a small perch and watch me play with another bird that was comfortable playing with me. The second time I played with another bird and offered her treats. On the third session I started actually handling her. She was very comfortable with me by the time I brought her home. I was able to put her into the cage she would be living in at home for a week before I moved her to my home. Once she got home, I didn't do anything other than put her carrier inside her cage and let her crawl out into the cage on her own. Then I didn't do anything else with her for a couple of days other than feed and talk to her.
All of that sounds pretty slow for a bird that was already really comfortable with me in the store, but senegals are known for phobic reactions, so with sennies it is always better to err on the side of caution.
Since you will not have the options of moving as slowly as I moved with my bird, and will have to bring it home before it is comfortable with you, be especially careful to follow the advice Jenny and Shandi have given. Perhaps you will be bringing the bird home with the cage it is familiar with, and if you can do that, that will be great! That will be one less adjustment.
IF AT ALL POSSIBLE, ask the present owners to put the bird into its travel cage and put the travel cage inside the cage in your home and let the bird climb out on its own. If the travel cage won't fit inside the bird's cage, try your best to give the bird another way to go into the cage on its own, such as holding the opened travel cage next to the opening of the permanent cage.
The nice thing about getting a two year old senegal is that it will have already passed through the most volatile phobic age, so it might adjust easily.
The draw back about getting a two year old senegal is that it will have already passed through the most volatile phobic age, so if the present owners have not known how to handle those well the bird may have developed some severe and significant phobias.
So my best advice is to really find out as much as possible about the bird and how it interacts in its present environment. If it is highly maladjusted, you might want to pass and wait for another one. There are many, many wonderful senegals and other poicephalus. There will be others available if this one is very maladjusted.