Hi, new to the forum. I have Nextel service with a plain flip phone. I need a blackberry. Can I buy one online, put my SIM card in and will it work. The cost to upgrade now is $479.oo, just looking for a cheaper way to get a Blackberry.

you will still need a blackberry data plan.

Wirelessly posted (Tmo's cheapest BB customer)

Do you use PTT? Not sure if you need the specific 8350i in order to use that feature.

You can buy used to save money. Best places are on forums like this, in the Buy/Sell/Trade section. Howard Forums has a lively market place as well.

I would avoid Craigslist as too many phones are stolen and no longer usable due to being reported. eBay is only slightly better. Read the FAQ about buying from eBay as it applies to all used transactions.

Buying a BlackBerry off eBay - BlackBerryFAQ

Also depends on your SIM. The 7100i supports the yellow SIM cards. If you upgrade to the 8530i you need to use the black SIM to support MMS.
You should be able to upgrade through Sprint for less than $479.00. Are you trying to avoid a contract extension.
You will also need a BB data plan.

It has to be a Nextel device. As Rambo said, make sure you read the FAQ article on buying a used BB on eBay. No matter where you by the device, that article might save you some grief.

Well I have a white sim card, so I guess this is not an option. I am in contract until April, and the phone will cost me $479, but if I just open a new account, I could get (1) for $149, but I guess my number would change.

And if you ended your current contract, you might owe the ETF as well.

No. When your contract has expired you can upgrade at the same price as a new customer.