Tips that I use:
Check for 98% feed back or higher. (this of course
is not set in stone because some sellers sell much
more than others and have much
more feed back.)
Look at the negative if not 100%,
sometimes negative feedback is
is unwarranted/retailation from a sour poose buyer or seller.

No what the item is worth.
Do not get caught up in a bidding war and end up over-paying.

Read sellers description/instructions carefully.

Make sure you know that the shipping costs are
before bidding.

Good communicatons is a plus and do not be afraid
to ask the seller any questions before you bid
on their auction.