A baised scale is one that is faulty so that it doesn't weigh accurately. This will normally mean that the seller has tampered with the scale so that it indicates a higher weight than the real weight, so what is weighed actually weighs less than the scale indicates. This is a classic way of defrauding the customer. The scale is biased in favour of the seller.

It could mean that the scale is out of balance; technically, not correctly tared (i.e. the zero value is not accurate).

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Note added at 14 mins (2016-04-28 11:39:26 GMT)
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Sorry; typo at the start: "baised" should be "biased".

It doesn't have to be fraudulent, though in commercial contexts (and metaphorically in legal contexts) a biased scale is one designed to favour one of the parties against the other. A scale can be biased simply because it's faulty:

"Bias: How can measurement be valid?
What if you find out that you always weight two pounds less on your scale at home than the one in the gym. The latter is one of the expensive balance models, and the gym attendants assert that it is accurate. Is your scale at home biased? It’s easy to imagine that it could be so without your suspecting it. Probably most home scales are biased in this direction!"
https://about.illinoisstate.edu/ktschne/Documents/CoursePack...

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Note added at 17 mins (2016-04-28 11:42:07 GMT)
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I suppose that even in this case it could be deliberate; the people marketing home scales could have biased them to make customers think they weigh a bit less than they really do, and thus sell more scales!