are all used LPs dusty?

Some are not but it doesn't matter.

If you want good results with used records, you have to be prepared to clean them properly first. There is lots of info here on A'gon and elsewhere already regarding various effective ways to do this.

Otherwise, stay clear of used records in general. Its hard to tell by visual inspection if a record is really clean or not.

Generally yes. The paper sleeves have a tendency to crumble and that leads to dust. A lot of good tips on cleaning posted, but don't forget steam cleaning as the best answer.

I use one of these. I use a carbon fiber brush for touch up before playing.This system works good for me.

1) Yes and no. New or used, all records benefit from proper cleaning. Cleaning not only improves the sound, it reduces the chance of permanent groove damage which can result from dragging a diamond chisel through a plastic groove filled with (microscopic) bits of junk. Much safer to drag the chisel through a clean groove.

2) Wet cleaning and vacuuming. Other methods provide some improvement, but you asked about the "best" method. Cleaning without wetting is an oxymoron. Wetting without vacuuming away the sludge just rearranges the sludge.

3) Cleaner? Probably. Clean? No. The dirt that matters is deep in the grooves. It cannot be seen without a strong microscope.

"Wetting without vacuuming away the sludge just rearranges the sludge. "

True vacumming is the most common solution I suppose.

I have found that using tightly folded edges of a very soft and absorbent paper towel like a record cleaner while the record is spinning does a very good job of removing excess solution when cleaning a record.

I've been doing this with good results using my Linn Axis table now for a good 25 years or so.

I use extremely dilute Shaklee Basic H cleaner in water, distilled water is preferred if availble.

I spary the record surface down thoughourghly, place teh record on teh table, give it a good scrubbing with an old discwasher brush, drying the discwasher brush periodically as I go using another absorbent paper towel as needed.

Then I use the paper towel technique above to dry the record.

Once you develop the technique, is is inexpensive and most effective with only one pass and only takes a couple minutes per record side.