On a recent bus ride, I saw a guy use his smartphone to shop for a Porsche on eBay. It might sound silly, but he's not alone. Apparently, shopping for cars and car parts this way is part of a growing trend' on eBay Mobile, 90,000 parts and accessories and 2,000 cars per week sold during the first quarter of this year, according to eBay.

This week, the company released a magazine-like app for the iPhone to cater to two markets: users who want to browse and bid on cars on the go and people looking to find parts for their current ride. While consumers can--and do--already shop for cars on the regular eBay app, this new app specifically targets the car crowd.

I downloaded the free iPhone app, called eBay Motors, to see what was on the market. While a tad out of my price range, the first cars that popped up on the screen were a Ferrari going for $200,000 and a Bentley priced at $156,995. If I was interested, I could have hit a button to place a bid. But I scrolled down to see other details like the make and model of the car and a button that would allow me to check the history of the vehicle.

Steve Yankovich at eBay told CNET he calls this situational shopping: "the trend of buying an item on mobile the instant you are inspired, and not having to wait until you are in front of your computer or at the mall. When a car enthusiast is working on their car or at a motors event, they aren't in front of a computer but their smartphone is handy."

It will be interesting to see if eBay can grab new customers by reaching out to them on their phones. According to eBay, the global growth of vehicle sales through mobile phones increased more than 160 percent this quarter when compared with the growth during the last quarter of 2010. The sale of parts and accessories was even bigger, increasing more than 250 percent. Online parts sales is growing faster than online car sales and the eBay app reflects that.

So customers browsing for Ferraris can see what's on the market at their leisure. An ad for a Mercedes-Benz SLS in Two Tone Mystic White caught my eye. So far, no one has bid on it, but with six days left on the auction, there's still time. If I wanted more information, there's an option to contact the seller directly to ask questions about the car. There's a button to ask for a free history report to see if the car has been in any accidents or to see how many people have owned the car.

The app isn't just for car shopping though, especially if you already own one. Users can add cars by model and year, or by VIN. It can connect you to spare parts you really need for the current ride. Once you have your cars plugged into the app, when you search for a desired part, like a water pump, underdrive crank pulley, or high-beam replacement bulbs, you only see parts that are listed as fitting your car.

The app is part of eBay's attempt to reach users outside its traditional online, bread-and-butter business, by going after more niche markets such as automobiles (and fashion) to get mobile customers to purchase stuff more impulsively.

When users do get that dream car, the app can let the world know by sharing purchases to Facebook friends and Twitter followers.

While the app is certainly convenient, I personally would want to take a Porsche out for a test drive before I commit. To be honest, I doubt anyone really buys a Porsche impulsively. But I can see people using the app to browse Ferraris they dream of one day owning, while shopping for car parts for vehicles they already have.