First off, don't sell yourself short... Even being a teenager you still have opportunities to make some money here and there....since it's summer you could always mow lawns to get extra cash....get a part-time jobs...do stuff for folks who'll pay you...

Depends on what kind of truck you are looking for...
Gas vs electric>
Gas tends to be more spendy, time consuming, takes more "know -how" (engine tuning), but for many it is more gratifying.
Electric is usually cheaper, usually just as fast, less messy, and is considered by most to be the ideal way to get into RC.

Stadium Truck vs Monster Truck>
This depends mostly on the terrain you want to run it on. either way you have plenty of options....
MT's are generally more spendy... most are 4wd and they have plenty of clearance and agility over most any terrain... the most popular are the Tmaxx (also Emaxx if you want electric) and also the Savage... Any of these run right around $400 (give or take $40- depending on where you buy it)
Stadium Trucks are pretty cool... Most are 2WD, they have less clearance than MTs, but are still considered "all-terrain"... I believe HPI has a 4WD Stadium Truck (it's called the RS4 MT) If you get a nitro one, it costs almost as much as a MT, but Electrics are a bit cheaper.... (the prices of stadium trucks can really vary so I'm not going to give any specific prices... except that they can run anywhere from $200-$700)

Good point...It's better not to trust ebay, by being wary you're more apt to get a good deal... lots of us have really gotten burned on ebay- usually because we trust them to be as honest as we are, or we simply didn't do our homework....... you can also look on the RCU marketplace for something... but even then you're still taking a chance on the seller's integrity...
My suggestion would be to do your homework... Check the feedback on the seller... don't bid if they have not had much selling experience (less than 100).... Also, be wary of sellers with negative feedback.... stay away from them... or at least check out why their feedback was negative... see how much that same item has been selling for recently so you know if you are getting a good deal or not (you can do an advanced search that shows only the completed items- then you'll be able to see what your truck has been selling for)... Only bid on stuff that has GOOD clear pictures that show detail.... generally this is where I've been burned.... I could'nt see the item clearly so I trusted the item description, but when it got to me it was all beat or broken. That really bites... Be sure you know EXACTLY what you are bidding on... if it says a Novak Rooster ESC.... don't go thinking it is the Super Rooster... When in doubt >Email the seller.
Sometimes you can win an auction with more than you want, sell off the other stuff and come out ahead...I once won an auction for a hopped up Tmaxx, a XXX-NT, a XXT, and a couple boxes of parts all for $400.... the pics sucked, so nobody else bid on them...I emailed the seller probably 10 times- he was probably sick of me by the end of the auction.... But I got the cars, sold a bunch of parts and the XXX-NT, and my overall out-of-pocket expenses for my maxx and XXT are about $100... I'm about to list the XXT on an auction and when all's said anddone I will have made money by buying that Tmaxx. The catch is, you have to have the cash upfront to do that...[&:]

Anyway... best of luck to you...