I know Dorking too, so know how small a place it is, but of course there are a few things which we don't necessarily know - 1). time of day - if it was early or late it could be dark, giving a certain degree of disguise 2). mode of transport, yes on foot it would be difficult to avoid being seen, but as my post earlier, if she was aided by friends could there have been a car right outside the shop for her to get in and out of?
3). CCTV - this is 1996, even at that time there wasn't a huge amount of CCTV even in large cities. When did the shopkeeper release the footage and when did the parents view it? The earliest quote from them regarding it is January 7th - that's a long time afterwards. Remember in 1996 the CCTV there was, wasn't kept for long and primarily on tape, which was re-used. If she did take a car journey then there wouldn't be any at the train station anyway.
4). On the subject of CCTV - 10 years after Ruth vanished, so too did Andrew Gosden. The entire CCTV investigation was messed up by the police, despite there being more of it. It was 3 weeks before they picked Andrew up on CCTV at Kings Cross, and by then subsequent footage had been lost.
Did the police take the sighting of Ruth seriously? As I've said, how long was it before they viewed it and got her family to view it? It hardly seems like it was quick. To be honest, did they take it seriously? Up until that point it seems that they were looking for a corpse rather than a live person, so it could've been just seen as another random sighting (these were coming in as far away as Canada) and being that it was on the anniversary, in the same town they may have not taken it seriously - as we know although they do a good job, many cases have been scuppered by the faults of the police.
Also this is interesting - Speaking on the tenth anniversary of Wilson's disappearance, Sgt Shane Craven, head of East Surrey police's missing persons team, stated that "In the weeks following Ruth’s disappearance there were some fairly reliable sightings of her in the Dorking area by people who knew her well." - was there any CCTV that was in operation that could've been checked by the police at that time, especially as a few weeks after it was 'still hot'?' There doesn't seem to be any mention of it, I think the likelihood is that at the time, and even by a year later, there was very little CCTV.
The newsagent probably had it because private businesses bought their own, and small shops were always at risk of shoplifters, but in streets it was still quite rare. Even the banks and building societies you mention would have had it only on the inside of their premises.