Based on my limited personal experience, I would make the following suggestions:
1. Buying slide rules (or anything else) over the Internet is relatively safe. Auctions can be good fun if you win, and frustrating if you lose. You might want to know the reputation of the seller before sending a lot of cash, however. Most sellers are very reputable, but a few aren't. Be aware.
2. The last minute is when everything happens on online auctions. Bidding in the last few seconds of an auction is known as "sniping".
3. When bidding in an online auction, remember you are competing at a global level. This means prices tend to be higher, especially when two newcomers get in a bidding frenzy. Decide what you consider a reasonable price, based on previous sales  - and stick to it.

The HP site gives some advice related to the use of ebay at http://www.hpmuseum.org/ebaywarn.htm. Although related mainly to calculators their advice is applicable to other purchases.