Absolutely. We took anything we wanted rid of. Anything that was unused for months (sometimes years)
High value items are going on ebay and electronics didn't sell at all. Everything was £1-£4 and everyone haggled, so if you want £1, go in first time round at £2. We were also advised not to label anything with prices as this turned people off.
General rule we found out pretty quickly. If they are interested they'll hover for a bit. If they ask you the price and you're too high, just ask them what they'd pay and negotiate from there. Being a sales person anyway, I loved it. I probably spent more time having craic (irish for fun/banter) talking with people, but that in its own way had more people at our stall.
People are like sheep, they just follow everyone else.
Be Seasonal. Heavy coats and jumpers sold the best. If I had 100 more, they would have sold no sweat. Saying that, my wife brought some swim wear and it went too.
Be Vigilant: Unfortunately not everyone has the moral fibre as us mustachians. Be aware of people passing fake notes and generally trying to pilfer your wares. We probably had too much out for a single day, so will probably do another car boot next spring and bring the remainder of our gear.