Budget: I try to set an annual budget for net spending, nothing that will get me into trouble, but enough that I can get some nice shiny things. If I want to increase my spending power, I sell some duplicates or extra stuff that has been accumulating. It seems to be a good time to sell right now, as I have been able to sell quite a few things since January. To make it stretch further, I also try to look for things that can be flipped at a profit. In a perfect world, I would be increasing the value of my collection and reducing my investment. But not doing that kind of volume yet.
Stretch: If there is ever something I really like at a good price, I will likely just purchase it and spend less later.
Regrets: Yes, today I went a little over my target spend for a nice 1924 penny (Canadian coin). On average it seems to balance out in my favor, so far.
Didn't bid enough: Sometimes on ebay, if I try to lowball my bid and end up losing by the next increment. Most of the time, don't really care as there will always be another deal come along. I do regret one lot that I was bidding on. it was a bunch of quarters with not great pictures, but I was 85% sure I spotted a tough variety (1906 small crown), I chickened out somewhere around $450, I think the group sold for about $900. A few years later, someone posted on here about their best win, I recognized the picture. They sent the coin off to PCGS
, it was worth about $12k, still regret that one.