Welcome to the forum.
Since you can't view the smoker beforehand everything will depend if they seller honestly answers. I couldn't even guess what a fair price on a used Bradley would be. Below are a few questions I would ask, and ask him to provide as much detail as he/she can.
First and foremost, ask if there have been any repairs performed on the generator or cabinet, and inquire if there have been any fires in the cabinet.
Most issues occur do to a malfunction of one of the components in the generator. Ask him/her if the bisquettes cycle at approximately every 20 minutes. Has the generator failed to automatically feed the next bisquette to the burner plate? Has there been any problems with the bisquettes cycling through immediately one after the other? A yes to either one of these questions could mean the micro switch, and/or the motor may need to be replaced. If you are handy, it is fairly easy to repair, but you should negotiate the price down, for your time and expense to replace them.
If the bisquettes cycle properly, ask if the bisquettes burn properly. If not the burner plate may need to be replaced. Yet another easy repair, but a negotiating point.
As for the cabinet, if it has the temperature adjustment under the door, ask if there are any cracks in the faceplate (the plastic molding that encases the temperature controller). If there are any cracks, moisture and/or grease can seep in and ruin the temperature controller. The face plate is easy to repair, though another negotiating point.
While talking about the cabinet, ask him if the heating element works. If the element doesn't work, then it could be one or more of several issues, and you may want to pass on this purchase. It could be as simple as replacing the heating element. But if the temperature adjustment switch is below the door that could be ruined, and need to be replaced. Or the problem could be the in-line fuse, or possible the high temperature sensor. All of these are minor repairs, but take time to troubleshoot, and if they are multiple issues with the cabinet, the cost can add up.