Hey there monika.
First, i know that the forums do not allow actual names of stores or avs to be posted (IE: when you named the person above) Its also against rules for the forums (As well as against tos in secondlife) To copy/paste private i.m's that you have had with other individuals, And im not sure if they allow the posting of abuse reports. You may want to change your first post to 'I filed an abuse report for a store because i feel the seller is a fraud' or something to that matter.
Secondly. This is actually a common inworld experience. A user pays and gets debited twice (This is SL lag by most means) However, if you paid twice i would assume you got double the product, so if you were buying something from her, you should have two items in your inventory that are exact copies.
I'm not sure what you had messaged her (Sorry im assuming the seller is a her, if it isn't i apologize) because you hadn't posted it, but if you came at her in an attacking manner, she may have been a little bitter towards you for coming off so meanly (Not saying that you had)
Most sellers design their work, and they spend a lot of time doing just that, designing in other programs. They also live in RL and can get up from the computer from time to time to eat, drink, run an errand, sleep, take care of their children or RL spouses as well.
The best rule of thumb is to message someone and wait. If 15/20 minutes go by with no response then send them a notecard (You can create a new one by going to your inventory then to create - New notecard) title it with 'Customer help' or whatever issue you are having, and retype your message to them, include all the details you can think of especially your avatar name. Then wait 24 hours. Most sellers will contact you within this time line, if they do not, you can either send them another notecard, or you can file a report. But remember that sellers are people as well.
Also, if a seller has a nice customer base, you can usually tell that they take care of their customers and are willing to resolve any issues.
I hope this helps with your situation. Take care and welcome to secondlife!