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 from a sales rep if their relationship with the owner is high, the item is cheap, they have a few customer loyalty stars, or the shop has a good environment score and they are in a good mood.
Some Sims are more susceptible to sales than others, while some may repeatedly turn down offers. This depends mostly on the Sim's mood and sheer randomness on the game engine's part. Finding out which Sims are good to try selling to is mostly trial and error, however the Perception business perks can be used to determine their susceptibility.
Sims can only perform the 'Basic Sell' action, moving the bar up or down by a fraction depending on the 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.
Sims can use the 'Offer at' option to offer their wares at a lower price to the Sim in question, the outcome being a bigger increase or decrease depending on outcome.
Sims can use the 'Hard Sell', which is similar to the 'Basic Sell' technique above, although there is a higher chance of failure. However, there is also a greater benefit for success.
Sims can use the 'Dazzle' technique, which can all but guarantee a sale - or lose one. A customer loyalty star can be gained or lost as well. A Sim who uses this technique will lose a small amount of energy.