There are loads of fake strings, mainly Yonex brands and mainly related to E-Bay. But unfortunately I do not know how to tell if the string is fake nor had any contact with fake strings.

And also, probably unfortunately for you, I have to agree with you that if it is too good to be true then it normally is! Meeting at a service station is a bit  and I would advise against it in my opinion. For the cost of petrol to get to the service station, you could get them to post it to you (well, before Royal Mail hiked up the price's anyway).

But at the end of the day, it's up to you. It could be a genuine seller who needs the cash and is maybe a bit weird (I say this as I can't see meeting at a service station going to well regarding turn up on time and finding the person and what not). 

I would stick to genuine sellers, which says a lot as I don't have much money so an opportunity to save some cash can't be passed but some times it's worth doing it properly for the peace of mind.

Just my 2 pence and a fluffy mint