I've bought a number of OOP models. I started my chaos army when I was in 7th or 8th grade now almost 20 years ago. I have an especial and nostalgic fondness for rogue trader era chaos marines, dreadnoughts (I have 3!), daemon engines (3 of them too!), and daemons. I have a lot of them from 'way back' but I've also supplemented my collection with models of that era from eBay. I've also assembled over the years bloodletter units from each era, 4 editions of bloodletters =)
My suggestions to you when buying OOP models:
- Do your homework, make sure that the model is really oop, and find out what game edition and year its was produced.
- Bide your time and don't buy right away.
- Unless you're sure you have an amazing deal, especially on eBay its often worth watching the secondary market for a few weeks or a few months to see what the going rate is. One week what you're looking for might be scarce and expensive, the following week there might be a ton of auctions for a lot cheaper.
- Set your maximum price before you make a bid.
- Understand what you want and what it is worth to you, and be ready to let it go if you can't get it for your maximum price.
- Auctions are designed to encourage snap judgments at the last moment - after all, people like to win things!