It would be 'at the trade fair stand' if we're talking about 'the trade fair stand' like a booth.
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Natalia09And what would be the alternative?Some kind of an elevated floor like a stage. If that's the case, then you can use 'on' or 'at'.
Natalia09Do you mean "a booth" is smaller than a "stand"?The size does not really matter. It was just an example for comparison to show that they are both separate area.