thanks for the reassurance about the price. Wow, it sounds like you got an excellent deal on your 520. there is a 1984 Trek 520 on ebay currently that I thought about but opted to go for the newer frame instead. it's a beaut, though, with the tan metallic paint. Well done!!
Is the Trek 520 all original equipment? If you want to check specs, google a search for '1999 Trek 520', and go to epinions.com site (should be first site showing) and check details. Also be aware that Trek had a recall of the ICON handlebar stems that year on their 520s. I also have an '84 Trek 520 that is just a great bike--that year they used a longer chainstay on the 520s. Mine was a thrift store find--$35, w/ original equipment & in excellent condition except for some paint stratches on the top tube. Sometimes you win the lottery!
Thrift Store finds like that are indeed rare. Congratulations.
$400 is reasonable out in the 'Want-Ads" if it's in good/very good shape. I'd be cautious about buying a bike off E-bay since you can't test ride it. You might be looking at eating some disapointment rather than hassling with shipping it back.