Most business operators in SL are honest and will try to make sure you get your product as soon as they become aware of the problem. There are however a few scammers operating that promise to sell you goods and never deliver. I became aware of one who set up vendors which were supposed to be selling subscriptions to porn sites, they took the buyers money, several thousand lindens, but deliverd nothing to them but a note card describing the product they were supposed to get. I became aware of this because the scammer was distributing a notecard that he had obtained from one of my products, so it showed me as the creator.
I had to take the unhappy customer to the location where he made the purchase to show him that the vendor he paid money into was owned by someone else. Someone who never responded to IM's, notecards or emails, he just took their money and ignored them.
If you are dealing with someone who is not well known in SL and seems to have a questionable business, it would be best to make your purchases only through the SL marketplace. At least there you have some chance of getting help with the purchase and if nothing else you can leave a negative review on the product.