Hey, first and foremost I'm going to tell you to be careful with buying/selling things on ebay. Their really sticklers about anything with the word bitcoin in the name. Additionally, if something doesn't feel right, back out.
That being said,
The KNC Jupiter is worth it depending on the price. You should look up some calculators
http://www.bitcoinx.com/profit/ is a good one and see what the cost is vs. the expected income. Be sure to look up the power utilization for it as well so you know what your getting into.
Just know that he is up charging by $1,000 from what you'd pay to KNC but you get it faster. Also know that this thing will be loud. However a simple look at the calc:
Cost: $5999.99
Power Consumption: 1W per GH/s (note: not ghz) = 550W
Power Cost: assumed $0.15 (get this from your electric bill, they usually show you the average USD/kwh)
Hash Rate: 550 GH/s (note that this can vary based on a bunch of factors but should average out to around this)
Note: the conversion rate used on this site is from Mt. Gox (IIRC) and it's running a touch low. Currently BTC-E.com is trading at $913.2/BTC and BTC China is trading at $1061.12/BTC so this field depends on where you are trading. The conversion rate only affects how "fast" you recoup your expenses see below for more info.
Mt. Gox conversion rate:
Potentially Recovered Expenses in: ~20 days
Potential First month net profit: 3243.83 USD
Potential Second month net profit*: 8868.35 USD
BTC-E conversion rate:
Potentially Recovered Expenses in: ~17 days
Potential first month net profit: 4666.41 USD
Potential Second month net profit*: 10233.52 USD
Bitcoin China conversion rate:
Potentially Recovered Expenses in: ~15 days
Potential first month net profit: 6265.16 USD
Potential Second month net profit*: 11767.77 USD
*: Note that the second month profit assumes a stable difficulty (with a low increase) and that the USD/BTC stays the same (which is unlikely, it will fluctuate). However this shouldn't discourage you as the possibility to mine using this is still great. Additionally the second month profit shows the profit from the second month ONLY. I got this by calculating one month, then subtracting the first month from the second to get the profit for the second month. The calculator automatically removes the cost of the hardware from the first month's profit.
So, as to your question, yes this is a good starting device IF your willing to put up with the noise and the potential risk in the market (fluctuating USD/BTC rates). Currently it is one of the better miners out there. If your willing to wait though you could go through butterfly labs who is making a PCI card that mines at 600GH/s. Both will require a fairly beefy power supply (pick one up from your local computer store or online). KNC recommends a 1200W power supply with at LEAST 4 PCIe power connectors for their November Models so be sure to ask which month this was produced in. The connector information will be shown on the specs for the power supply, and I recommend you spend the money on the 1200W version should you get this.
If you opt for the butterfly labs PCIe card, be sure to have at LEAST an 800W, possibly more if you want to run additional cards. On a side note I don't recommend the PCIe card for many reasons that are merely technical in nature, however I will warn you that butterflylabs has a rather odd track-record with the BTC community as many people are waiting for their orders from a while ago and now their about to ship these new ones so it's a 50/50 that they'll hold up (they ship just REALLY slowly).
As for lasting for months to come, I can't say for sure. With the rate the global hash rate is jumping and with the difficulty rising you will most likely be buying another to keep up quite soon. The good news is that the longer you have the first one mining the more money you can make which could actually pay for another one of these in just a few months (even with rising difficulty).
I hope this helps,
AJ