No certain value, just >0, but of course, the more civilians, the bigger chance. Radius doesn't affect that directly, but with low radius easier to spot special civilians around, because all of them are close. But it gets crowdy and weird then, I play with max amount and max radius for example. 10% means ten times lower chance than for 100%. Exact calculation per civ is complex:   As long, there's less than 6 of "blue mark" civs around, chance for one per each civilian is 5-25% depending on reputation, tested once per civ, when you'll be closer, than 100 meters from him. Amongst each of "blue mark" civs in 1000 meters radius around may be at once up to one doctor, one mechanic and seller, where for each of "blue mark" chance to become one of service providers is 0-50% depending on reputation (0 for extremely low rep, <-125). Then it is tested, which service exactly: 70% for the doctor, 15% for mechanic and finally another 15% for the seller, if there's no this kind of service around.   Simplifying, for reputation neutral, 0, there's (0.15x0.25x0.15)x100%= 0,5625% chance for each civilian to become a seller, which means one seller per 178 civilians statistically. Designated service provider will become a seller also if there's already one doctor and one mechanic, thus probablity will be a bit higher if I'm not mistaken. 
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