This is a good topic! I am laughing already because I also love the chase.
I love hunting for them, finding ones I want and bargaining. Yeah, I generally have an idea what it's worth to me but a lot of factors can play into it. I have paid more for machines because the seller's heart was breaking and it belonged to their mother. I figured I was buying good karma as well as the machine in that case.
Other times I really look them over "muttering" lol..."oooh that doesn't look good" and am honest if the machine needs new wiring to be safe, the oils are really vanished, parts frozen etc. I'll tell people if they are interested I know the place in town charges 60 plus parts and they could have it working again..but if they don't want to bother I will take it off their hands because I like to tinker. I will offer 5-10 bucks. I admit to "tinkering" but don't like people to know I collect them for fear of losing my bargaining edge or them thinking I'm crazy!
I avoid even dealing with those people quoting eBay prices or showing ads.. I offered a gal in town 20.00 once for a crinkle 99 in a bentwood case and she was highly insulted and showed an ad where they were worth much more on eBay! She herself paid 100.00 for the machine a year ago...but didn't even know if it works or how to use it. This was over 6 months ago and she still has the machine even though she has lowered her price to 20.00 now.
It's a balancing act because I have also told people they were selling too low. I haven't shared my Featherweight story yet but I answered a CL ad from a guy who was clearing out his mothers things and found 4 old Singers (ALL featherweights!) and didn't have a clue what they were worth. His ad said he would rely on the buyers to offer a fair price. I almost died! I called him on the phone and explained they were in high demand and what they were called and said he should do some research... the machines he had should bring at least 300.00 each. He was amazed and said thank you. The next day he sold me one for 145.00.
I chatted with him a long time and he was so fun and he also gave me a box full of quilting magazines for free.