the only software you should purchase these days is in a shrink-wrapped package.  before you place a bid on one such package, go to the web site for the company that puts out that particular software.  you might find that there is a newer version, or a version that has more of the features that you want.  also, the manufacturer/software writers themselves often offer their software with a discount.  find out that price.  then compare it to the same software on eBay.  use the advanced search, completed listings page to find out, really, how much you should expect to pay (green prices) for a given software package.  if the company that makes it is cheaper, buy it from them, but of course, check on taxes and shipping.

i have oftentimes, via doing research on software, found forums that discuss the pros and cons of different brands of one type, for instance, antiviral software.  a wonderful review site on the net is CNET.  but on top of that, use a search engine to find reviews of what you want.

be sure that you will be able to register the software so that you can download updates and patches to it from the manufacturer.  you SHOULD get a registration number on the paper holder of the CD-ROM that you get, which you really have to keep in a safe and handy place.

too much software is pirated or illegal.  so, ask your seller many questions.  if you get vague answers, or something like this:  “just buy it and return it if you do not like it,” be very careful.  you are going to have to pay postage for a return.  do you want to?

i often find that when i watch the sales and bidding that i can buy the product on a BIN and actually save over and above the winners of the open bidding for the same product.  but you have to do your homework.