If you're considering buying a 1992-1998 Seca II/XJ600/Yamaha Diversion, here are some things to consider:
Most Seca II's are 20 or so years old (the newest model sold in the US was a 1998 model, making the bike between 15 and 20 years old), and do often show their age. Keep that in mind when looking at one and set your expectations accordingly. These bikes are easy to work on, parts are relatively cheap, and maintenance is easy to perform. The expensive parts to repair on a Seca II include the fairings (front fairing, windscreen), tank, cylinder head and exhaust.
Going rate for a used Seca II in Denver is between $1800 and $2500. A bike which has accessories (center stand, belly pan, engine guards, etc) should be slightly worth more than one without as these parts are hard to find. Paying more for a bike with good paint and cosmetics is worthwhile. A bike that is in good shape but that requires maintenance should still be in the $1800-$2500 range. Maintenance is an age and mileage condition and should be done by the new owner as part of any bike purchase. When buying a used Seca II, keep in mind, these bikes sold new for between $6,800 and $7,000, depending on year and locale.
When starting a cold bike, expect to use the choke (enrichment) on the lefthand side. The bike will take 10 minutes or more to warm up before it will accept much throttle. This is quite normal with the XJ as they are jetted lean from the factory. This effect will be reduced considerably if a previous owner has installed #20 pilot jets. As the bike warms up, on enrichment, the engine revs will increase. As they near 3k, click the chock to the second position and the idle will drop back down. The bike should be rideable in the middle or off choke positions.
Start the bike and listen to the sounds the engine makes. A quiet motor is good. A noisy motor isn't bad, necessarily. These engines make quite a racket on the top-end and oil-pump noise when the clutch is released (idling in neutral). Don't be afraid of these noises as they can be remedied with valve clearance adjustments, carburetor syncs and clutch basket pin replacement. Noises heard on the bottom end (listening below the sump) are typically bad and may signal lower bearing issues. This type of damage is NOT common on the Seca II, however.
When cranking the bike, listen to the starter. It should engage quickly and smoothly. If it spins over (without turning the engine over) or if the bike has to be push started, beware. This likely indicates a starter gear failure. This results when the bike is cranked with fuel in the cylinders. The fuel won't compress and the starter strips the idler gear instead. Starter idler gear failures crop up more frequently than other problems, on the Seca II. If the starter idler gear has failed (sheared teeth), to replace it will require removing the engine and splitting the case/removing the gear train. Yamaha has released updated parts for this. A bike with a stripped/failed/broken starter or starter gear is worth significantly less than one without a failed gear. At best, a bike with this problem should sell for $500 to $700 provided the rest of it is in good condition.
Look at the bottom side of the engine for oil leaks. Typical oil leak locations include the valve cover gasket, oil galley plugs, clutch cover gasket and sump gasket. The valve cover gasket is easily replaced but is expensive. The sump gasket is tedious to replace in situ and should be done as preventative maintenance any time the engine is removed. The other gaskets are cheap and easily replaced. Oil leaking around the cylinder head is not common but can be remedied by replacing the head gasket and oil-galley O-rings.
Inspect the underside of the exhaust for damage or dents. Stock exhausts do show up regularly on EBay without rash or dents, but they do sell for a premium in that condition.
Look at the chain and sprockets. A chain that has been maintained will be either clean or well greased, but will not show rust. A rusty chain shows lack of maintenance and should be replaced. Also, look at the teeth on the rear sprocket. The teeth should be shaped like a capital A. If the teeth are worn into a hook shape or are sharp and pointy, the sprockets should be replaced. Anytime a chain is replaced, both sprockets should be replaced as preventative maintenance, and vice versa.
Look at the front forks and verify the condition of the dust and fork seals. If any oil is observed on the fork tubes, new seals will be required. Replacement seals are cheap but replacement requires complete disassembly of the forks; fork seal replacement and fork oil changing is part of routine maintenance.
Sit on the bike and notice how much the rear end sags. When riding the bike, pay attention to how the back end feels over bumps. The rear shock on the Seca II was soft when new and they don't age particularly well. Replacement (VStar and FZ6R) shocks are readily available.
Look at the tires. They should have adequate tread and should match. Tires should be easily dented with your thumb nail. If they are hardened, oxidized, or showing sings of cracking, they must be replaced. New tires typically run between $150 and $250 for a pair, in stock sizing.
Look at the steering stops for signs that the bike has been dumped. Along those same lines, look at the cylinder head fins and see if they're ground down. Also, look at the frame, where the headset meets the frame itself (below the VIN sticker) and check the paint for cracks or other damage that may indicate it's been crashed or is otherwise bent. You should walk away from a bike which has frame damage.
Look at the front fairing. Make sure it has one and that it is in good shape physically (paint is easy to redo). Make sure it also has the windscreen and associated hardware. The fairing and screen are expensive to replace if missing. They can be repaired using ABS cement and this is relatively easy to do.
Fuel tanks for these bikes (in good shape) are getting quite rare. Look at the tank and ensure you can live with any damage it has. Pop the fuel cap and look into the tank. Rust on the bottom of the tank, near the petcock, may require the tank to be welded; this can be difficult to do and a tank in this condition should be replaced as soon as possible.
Because the Seca II can generally be had for less than $1500, it is not uncommon to find them in heavily modified condition. Be very careful when considering a bike that has had extensive modification as the quality of a post-modified bike is strictly contingent upon the skill of the owner who made the modifications. Bikes which have had their frames cut or other structural modifications should be checked thoroughly for quality of fabrication, welding, etc. Walk away from a bike which has been poorly modified as restoring such a bike will require a big investment in money and skill. Unless a modified bike has been done with great skill and the modifications are exactly what you want, a modified bike will ultimately cost considerably more if you have to repair the modifications or replace components. A modified bikes typically sell for less to considerably less than KBB.