First, let's look at buying used car audio equipment in general. This is risky. There is almost nothing in car that doesn't have a very fixed and limited life span. The worst are subwoofers and CD-players. Subs obviously take a beating every day, and it is nearly impossible to tell if they have been damaged internally, until they fail completely. CD-players suffer from contamination from dust and dirt which impedes their ability for the laser to track the disk properly. The first sign is more frequent skipping, then the ability to detect disks becomes limited. Also, the laser and transport mechanisms do fail and are usually most of the cost of a replacement unit. Think wisely before buying online. While it may be a good deal in the short term, over the long run, it could end up costing significantly more than buying new.
Now for buying new products online. There are very few topics that I take such a firm stand on, but this is one of them. While I have enjoyed the 5+ years I have spent on the net, not every aspect of it is beneficial. One of the biggest problems with the mobile electronics industry is the lack of competent installers and knowledgeable salespeople. Many installers don't have any formal training in the Car Audio industry, but thanks to schools like Mobile Dynamics and MECP certification, this number has been decreasing at a steady rate.
It is the policy of most major manufacturers to NOT SELL their products online, in any form. All offer the same reasoning, and there are some additional reasons I will enlighten you to.
Selling online is purely a numbers game. The cheapest price for a specific product always wins. This is good for the consumer, but terrible for the dealer network.
In the event that you need technical support, many of the 'online stores' have little or no formal training, and little experience in installation. You are on your own. Improperly installed, or improperly used equipment is the most common cause of systems failures, and poor performance. If you have a simple question, need a spare or replacement part, this will be unavailable from many online sources.
Selling products online typically violates sales territory agreements that distributors have with their dealers. All companies set specific regions for their sales reps, and track the sales number carefully within this region.
In most instances, high quality products have NO WARANTY if purchased online. It doesn't matter what the 'store' says, it is the policy of most companies to not sell online. This is the biggest reason why you should not consider purchasing online.
Before you buy online, contact the manufacturer of the product you intend to purchase, and ask them if it is covered under warranty, it's worth a bit of research to save a lot of headaches later on.
Many manufacturers sell their products to distributors without warranties. It is then the responsibility of the distributor to warranty the products. This means that a manufacturer warranty may not be transferable across borders.
If you purchase something like an amplifier for example, you may save $50 as compared to buying it from a local dealer. However, should the amp fail and require warranty work, you are likely to spend more than $50 in shipping and potentially duty charges (if it's across the border) getting it repaired, plus it will add several days to the turn-around time on the product.
You are welcome to purchase your products from any source you want, but I have always dealt directly with local retailers, and more often than not, established great relationships with those shops. Having them as friends has saved me several times in last minute, time-sensitive situations where I needed a favour.