First of all, DO NOT buy a rope or ANYTHING for that matter that you will trust your life to, purely based on what a single salesperson says, or what I say. Find a few experienced climbers (at least 5+ years of experience each), preferably certified guides and ask them their opinions. Now on to your question:
There is ambiguity in this because apparently ropes are labeled by their percent elongation for 'static' and 'dynamic' testing/use, in addition to themselves being called static and dynamic ropes. That is, you can have a rope that is static (meant for top roping, hauling, etc NOT for lead climbing) or dynamic (designed for lead climbing) and any rope from either category can be testing for 'static elongation' and 'dynamic elongation'. i.e. static ropes will have crappy dynamic elongation by definition.
See the petzl nomad for example:
Static and dynamic elongation is precisely defined here:
In other words you should ask the salesman if he is talking about static or dynamic elongation. 10% dynamic elongation is certainly not enough for lead climbing considering the Nomad has 30% dynamic elongation and that is certainly a lead climbing rope.