I've been pretty good about jumping on anything that seems right to buy, even if it is more than I'd like to pay. I can't think of many missed opportunities that I willingly walked away from. You might count DS games, since I remember when they were still plentiful at GS, now most of the good ones are hard to find with the box and manual. However, many of them were bought up long before they were heavily discounted. Wasn't willing to pay full retail for most of them.
Now one instance occurred to me, Conker Live and Reloaded. Passed up a $10 copy back in 2009, and regretted it a lot. Ended up finally finding a copy last month for $5 complete. Suffered years for that mistake, but I was newbie then.