You should never buy any vehicle - two or four wheeled - without seeing it first and checking it over.
If the bike is too far away for you to go and view, find one closer to home that you can inspect before you bid.
Ebay, Craigslist, a dealership
CRAIGSLIST
Snowboard because its better than a dirt bike.
50cc dirt bike may be available at a bike specialist store, but very unlikely. Most 50cc dirt bike are seen online for purchase only. For example eBay and Amazon.
Probably at a garage sale or check on ebay or craigslist
DiomandBack Groove
That low I'd say ebay, craigslist or a garage sale is your only option.
To make an electric dirt bike go faster it is simplest to buy a motor that can support a bigger battery and then get a bigger battery. Now you should lighten it. Also, don't be cheap, buy a dirt bike with a gasoline or diesel engine.
When riding a dirt bike, one should wear the safest gear possible. There are plenty places where you can go buy some authentic dirt bike gear. Try going to the online market BTO Sports, they provide all information on their clothes and you can buy for a good price.
Buy a USED 250 dirt bike.
For a 2-stroke dirt bike all you have to do is, buy a taller cylinder, and buy a longer crankshaft.
Not usually. Some dealers will give you a helmet when you buy a bike, but nothing else.