It can depend on many factors, time, size all sorts of things. If the spread is really tight..a couple of pence, it can cause issues as to what the actual trade you made was, and anyway, what you Sell someone else Buys.
no one needs to keep any faith. That's the sort of comment one makes when bad things have happened, which hasn't. I take it it's not a phrase you know the context in which to place it?