Just buy from a person with a high rating. Saving $300 is a huge difference, I usually would say support your local shop but $300 is alot. I have bought a Viper alarm from eBay before and had no problems. I would look into Autopage or Compustar tho IMO.
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MECP Certified.
Alarm: Viper 5200
HU: Stock
Active Set-Up
Tweets: Image Dyamics CD1pro mini horns
Mids: Sundown Neo Pro 8s
Amp: Kicker IX500.4 (2)
Sub Stage
Subs: (2) Dayton Audio 15" HF
Amp: Kicker IX1000.1