Sales
Some items just fly of the shelves of a sims business. Others need a little persuasion. If a sim has a high Charisma skill bar they will earn badges faster.
The sales bar
The sales bar will appear above the sims heads when they are looking at an item, the direction the circles around the bar are moving determines whether the sim is deciding for or against buying the item. If ignored for too long a sim may get annoyed and a customer loyalty star may be lost. Sims may buy without any interference of a sales rep if the relationship with the owner is high, the item is cheap, the have a few customer loyalty stars or the shop has a good environment score and they are in a good mood.
Susceptibility
Some sims are more susceptible to sales than others, they may repeatedly turn down offers. This depends mostly on the sims mood and sheer randomness on the game engines part. Finding out which sims are good to try on is mostly trial and error, however there is a Perk which can be used to determine their susceptibility.
No badge
Sims can only preform Basic sell, moving the bar up or down by a fraction depending on outcome. Sims can also use the 'Sales...May I help you' option on a sim that looks confused. Outcome depends on whether the item is in stock or not. Finally any sim can sell their pets.
Bronze
Sims can use the 'Offer at' option to offer there wares at a lower price to the sim in question, the outcome being a bigger increase or decrease depending on outcome.
Silver
Sims can use hard sell, similar to the techniques above, although there is a high chance of failure.
Gold
The technique dazzle is unlocked, which can be used to either guarantee or lose a sale depending on outcome, also depending on outcome, a customer loyalty star can be gained or lost.