I'm an introvert and I've done sales and customer service and loved both, but wasn't too keen on some of the teams I worked with.
The pressure and targets were fine, talking to customers was fine because I focussed on professionalism not my personality! But sometimes the internal competition wasn't good because other people took my introversion to mean that I'm stuck-up and if I congratulated them on sales, they took offence, if I didn't loudly celebrate my own sales, they took offence etc.
It's definitely not for everyone, but I can honestly say being an introver may well make you pretty good at it, if you can find the right sort of environment to do it in!