[Gummerfan]

I bought our Trailmasters two years ago for Christmas. A 150xrx for our 10yr old (had to add some blocks to her pedals, but she loves the buggy) and a 300xrx for, well, me. We bought ours through a local motorsports dealer and they did an outstanding job on the assembly, including changing all the fluids. I don't even know offhand how many hours/miles the 300 has on it (too lazy to go out and check the meter right now lol) But other than replacing a hub just recently I've never had a problem with either buggy outside of routine stuff. My wife takes the 300 down the road to her fishing hole at least 4 times a week.
When putting it in gear, are you revving the engine? Let the engine idle, put it in gear, put one foot on the brake and s l o w l y apply the accelerator until the clutch engages.
If you do a web search for "300xrx manual" or "300 xrx parts list" you can find some links to downloadable manuals in pdf. One is done by an owner and is better than the factory manual since he includes his own observations and experiences.
Funny thing about the taillights. Mine frequently blows the right (passenger side) brake light. I noticed the last time that the inner bracket was almost broken in two, probably a combination of flimsy steel and vibration. I built it back up with some steel epoxy putty and so far, so good. One owner bought a set of trailer lights and just replaced the stock units with those.
And yeah, the 300 is a bit too much buggy for an 8 yr old. The 150 would be a better fit, and it's a fun buggy for grownups as well. I get a giggle when I drive my daughter's 150 because it's a lot lighter and more nimble than the 300. :-D