If your budget is dead set on $100 the 970a-ud3 will really be the way to go. Especially if you want/plan to overclock.
with all of the suggestions.....
the ASRock970 extreme 3. Great board for a budget board if you are planning on building a computer and running it stock. I used one in my brothers computer build and it has been going strong for almost 2 years without a single hiccup. I have him running an athlon II x3 455(Rana) unlocked to a quad core, but at the stock voltage and stock clocks(1.32v or something like that under load). However, for higher tdp processors or if you plan to overclock that power phase setup is a bit lacking. And although it is heatsinked, without direct airflow, those suckers do still get a little toasty.
ASRock 990fx extreme 4. Great board at a great price with great functionality. matches up quite nice with/against gigabytes 99fx UD3/UD5 boards IMO. Can't go wrong with it.
ASUS M5A97. This board is almost identical to the ASRock 970 extreme 3. When I was building my brothers computer I saw little tmo no difference between these two boards. Both had the same functionality. Except the Asus board came with a beefier VRM heatsink and a higher price. Just like the 970 extreme 3, this board is a good budget board and will allow you to do some small tweaking, but I definitely would not look at it if I was planning on doing any type of heavier overclocking.
ASUS M4A89GTD Pro...I don't believe this board supports am3+ cpu's does it?
Biostar Ta990fxe. Another board that isnt bad. Biostar definitely kicked their quality up a little with this one. Decent board and gives you all the function you need. A fair choice, but for the money there are better options out there.
This is just my
on the topic and the board suggested so far, along with some personal experiences with the products.
I would say stay away from ebay for purchasing computer components BTW.