Just be patient. Set up eBay alerts so you know when new items are posted. Don't forget that PayPal does "pay in 4", if you need to break up some of the more expensive games into easier to manage payments. Games like tank comment, you're just going to have to accept that you won't get a deal and will have to dish out a few bucks. Sooner or later, a buy it now listing will pop up and you won't have to worry about being outbid.
In the Grand scheme of things, the 7800 is a very doable and still fairly affordable library to complete, compared to most other consoles.