Gaelen,
Just speculation since I can't put my hands on it, but the frame may be a boat trailer.
The wheels are small, indicating a lightweight axle.
Fairly lightweight, especially if it hasn't been reinforced.
The "Oak" appears to be Oak plywood, it may be only 1/8" which is also light.
No galley or cabinetry=less weight too.
Perhaps the biggest reason for the light weight is that each of us has a different "feel" for what something weighs.
The owner states he is "guessing under 500 pounds".
It is easy to pick up a tongue and guess the weight, but dermining how much the entire unit weighs is well, just a guess?
Your idea of buying a trailer and then adding or redoing things to suit you is not a bad one, and is done often, though more often than not, it is a lot more work to undo someone elses efforts, and then make it the way you want it than to just build what you want to start with.
A compromise many find suitable is to buy and use someone elses creation, decide what they want different, and then sell to finance their "dream trailer".
The fellow selling this one has lowered his reserve...as long as someone understood just what they were getting for the money...it could make someone a good starter trailer...if they could use it "as is".
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...