Any auction, be it e-bay or an in person auction, always follow these rules.
Find the items in the auction catalog, if it's an in person, or other type of online auction.
RESEARCH-RESEARCH-RESEARCH! not just online searches, though that's a good place to start. Try to get an idea of the age of the item. Go to your local library and dig into the microfilm. See what similar items sold for brand new. Use an inflation calculator to figure out what the price would be now.
If you can find ANYTHING else about the use of that item, check that two!
Condition is king!! Try to get an idea as to the condition of the item BEFORE the auction and base your value calculations on that. Look carefully at kit when you get to the auction Sometimes you will be disappointed, Be ready to walk.
Know how rare the item is, but if it is that rare, you are not the only one who has lived without it for this long. You can continue to do so!
Know the value and have a topside that you refuse to go above.
Know whether or not there is a reserve. If there is, and you agree with the seller that it's worth AT LEAST the reserve, don't even throw in your bid until it gets to just below reserve, then bid just above reserve.
If there is no reserve, pay attention to what similar items have sold for recently. That is your best guide.
Know in your mind the realistic, fair value when you walk in the door. Only bid when it gets close to that. Within 10% or so. Then bid ONCE and if it goes more than 2 or 3% above that WALK!!
YOU WILL FIND ANOTHER ONE no matter how improbable that seems!
While I have never bought enema gear this way, I HAVE furnished 3 children's bedrooms for a grand total of less than 6 months combined salary for both of us!