How to use vendors/es
All vending machines look and work slightly differently, but most have a commonly understood set of behaviors. Typically they have arrow buttons to navigate forward and backward. The vending machine will usually indicate the price of the current object you are viewing.
In most situations, you right click and Pay the vending machine the specified amount to buy the shown object. After doing so, the object will be dispensed to you, but you must be sure to click Accept to take the object into your inventory. If you do not Accept the object (i.e. accidentally hit Decline), the vending machine will not know. In that case, you'll need to use the vendor's 'redeliver' feature to ask for it again (if it has one) or you will have to contact the item's creator to receive the copy you just paid for.
The item you purchased will be delivered to your inventory in one of two ways: a folder most likely named for the item you bought, or a box in your Object folder. Check both locations.
Due to the prevalence of re-sale vendors, be sure you know who the item's creator is before contacting someone with product questions, as it is quite likely that neither the vendor creator nor the vendor owner will know anything about the items the vendor sells.
For help with specific brands of vendors, refer to the vendor creator's documentation. Some are listed below for your convenience: