That looks like a nice solid wood stand. The stand alone is worth $300+ new. <If it is actually particle board made to look like wood, than cut it down to ~$100.>
If the tank is in good shape, and the filter & lights are good quality, $300 can be a steal. It all depends on what is included. <Ask the seller brand, model, age & other details on the filter. For light, ask what type it is (T5, T8, PC, etc), wattage & number of fixtures/tubes.>
As for the tank age, a couple ways to tell:
1. Some new tank (mine does) has date of manufacture stamped on the bottom. That is the easiest way! <Mine has a sticker in the inside of the trim so it is temper proof, and you can only see it through the glass.>
2. Failing that, you can check the shape of the seals. Silicone seals dry out & get brittle with age, so a really old tank (10, 20 years) might have seals that is lifting or cracked or feels hard. It is not a big deal, as it is pretty easy to reseal the tank ... but you can talk the price down in that case.
3. Check the plastic rim on the tank --- old plastic can be brittle/discolored. <And rims that are not black is prob. older ... as the beige & other non-black trims are more popular in the 80's or 70's. Of course this is not 100% reliable.>
4. Check glass thickness. Older tanks are not tempered & will use glass that is ~1/2" thick. Modern (big) tanks are generally tempered & the glass thickness will be ~1/4-3/8".
Personally, I don't worry too much about the age. Glass is ageless as long as it is well looked after. So the number of scratches is more important. You can easily make an old tank like new by resealing & buying some new trims, as long as the glass is in good shape.