I have purchased from Aquabid in the past but not bettas. As with any auction site, use caution. You are buying for an individual who listed on the site, which means you will need to pay attention to where they are located, their history and comments from others who have purchased from that person, etc. Some of the sellers are great... some of them are scams... some of them just inexperienced people who are trying to make a buck... etc.
One of the things you should pay special attention to is the location of the seller. If you are located in the US, keep in mind that sellers from overseas are not figuring in the fact that everything must first go through customs, which not only takes more time and often times inexperienced handling of your fish, but the customs charges as well. While the fish may be perfectly healthy when it leaves... there is no guarantee it will survive customs, and what looks like a great price may be extremely expensive once you figure in the charges for customs. Also be sure they have a permit for export or the fish may never make it to you.
I have heard horror stories about bettas from Aquabid, but then I have also heard of some successful purchases. All you can do is use caution.
What I ordered were live plants, and from someone within the US. The plants were great, however... when I opened the box the day it arrived, I was presented with loose fiberglass insulation, which is an extreme health hazard. When I wrote to the person about it I was given a "flip" comment that nobody had ever complained about the insulation before, so she saw no reason to change her packing procedures. I then posted negative feedback for her on the site, which brought some very hostile emails from her insisting that I remove the negative feedback.
I mention this because watching the customer feedback can tell you a lot, and also to let you know that anything is possible, you just never known til something arrives. That was the last time I ordered from Aquabid.
Hope this helps you.