Refinishing is DIY-able, but can be quite difficult, depending on the shape of the wood. The stuff you use to strip polyurethane is absolutely disgusting. And getting stuff professionally refinished can cost quarter to half the price of buying new.

Secondhand is still great and I would recommend it, but it's important to know what you're geting yourself into if you plan to refinish. Touching up or painting over is much easier than re-staining, though (but not as nice of a result).