just my opinion,
$400 seems kind of high for a 3 reel mechanical slot machine. I can't tell which manufacturer that machine is, but if it is a lesser-known one then you could have trouble finding parts for it down the road if anything ever fails. Find out the company that made it (IGT, Bally, Aristrocrat etc) and possibly the model number. There should be an idplate on the machine somewhere, often on the same side as the pull handle.
Keep in mind this is just an opinion, if you love the machine then it is worth almost whatever you want to pay. But you said you are new to slot machines, so it is best to be cautious because it is easy to make a mistake like overpaying or buying an orphan machine that there is very little support or information for it.