When giving a seminar talk, should I sit or stand?

I personally find that sit vs. stand strongly depends on the degree of formality and interactivity of a presentation.

• When the presenter stands, it signals a strong differential between the roles of presenter and audience (who sit). This generally creates a much more formal atmosphere and means that the audience will not contribute to the discussion except to ask occasional questions.
• When the presenter sits, it signals a more equal discussion environment, where it just happens that the presenter is the one who is structuring the discussion and/or controlling the slides. I almost never see this except in the most informal sorts of meetings within a project or group, but in such meetings (e.g., coordination of a project team) it is very common.

For the specific case that you give, of a department seminar, I would strongly advise standing. The reason for this is that, even if it is a fairly intimate and informal group, a strong motivation for the organization of such seminars is to give you practice for more formal talks. You may find it more comfortable to be sitting but this is a good opportunity to practice and find ways to be comfortable when standing as well.

Two caveats to all of this:

• If a person has a medical condition, they should take whatever position the medical condition encourages. I've stood and walked around at the back of a talk when my back was having issues; nobody expects Steven Hawking to stand and speak. If the condition is not obvious, you may wish to inform the audience so they know not to misinterpret your position.

• If you are giving a presentation remotely, the position that you are in generally does not matter, as a well-adjusted camera will typically only pick up your head and shoulders.

I have never seen anyone sit down to give a seminar talk (or even a lecture that I can think of), it would come across as extremely unprofessional (unless there were some sort of issue that restricted the ability to stand).

Your ability to feel relaxed and to concentrate on the talk should not be an issue: your talk is prepared ahead of time. What is important is your ability to interact with the audience, to command their attention and to draw them in to your presentation.

I don't think there is any question, and indeed, people have told you that you should stand; so stand.

I remember attending a mathematics conference. One lecture was presented by John C. Oxtoby. At that time (because of muscular dystrophy, I guess) he was confined to a wheelchair. But for his lecture he managed to lever himself out of the chair using crutches; lock the knees of his leg braces; and deliver the lecture while leaning against a pillar. (He had an assistant writing the appropriate things on the chalkboard as he spoke.)

He clearly thought standing was the proper way to do it. And I doubt your difficulties in standing are greater than his were.