I haven't been served yet.

Or: waited on

Or: Nobody paid any attention to me as a customer.

OT: Happend to me recently in France and Germany. Great countries what service and business sense is concerned. :-))

Oh,I forgot, French stores are typically closed between noon and 3 P.M., But it's a great country, one small problem: too many French. :-))

There are probably regional differences, and it may also depend on context. If the salesperson asks 'Have you been waited on?' then you could say

No, I haven't been helped yet.
No, I haven't been waited on yet.

But really, just 'No, I haven't' would be enough.

If there's a cluster of people and the salesperson doesn't seem to know who's next, you could use a progressive tense:

Excuse me, I'm still waiting to be helped.
Ma'am? I've been waiting ...

To me 'served' sounds a little more like food, maybe the counter of a fast-food restaurant or something.

'Nobody paid any attention to me' could be a complaint to the management, or a review, but to my ears it's not really what you would say at the moment when you're trying to get the salesperson's attention and buy something.

—> Great countries [what] where service and business sense [is] are concerned.