Generally in my experience to get a 'sea trial' there has to be some money on the table.. even in a private sale. Unless you're a very giving person with a ton of free time, giving a 'ride' to every looky-loo that comes along could become tedious and time consuming.
Rather than targeting sellers, try to find a crew position for a local YC's race program, you'll meet some people and make connections that may also lead to some crewing opportunities on less performance oriented boats. Bonus: you can sail for 'free' (less the cost of an odd case of beer), learn tons and if you're reliable you'll get called back for more. Many casual fleets are often short handed and willing to take crew on here and there.
Just be honest, upfront and dependable. As you learn you'll likely be put in positions with more responsibility and learn still more. Then when you finally go shopping you'll be in a much better position to judge what you really like/want/need.....
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"