I used to buy lots of my stall stock from boot sales, so I was one of those pushy early bird dealers who clustered round sellers as soon as they opened the boot of the car. I have never sold at a boot sale, but are my tips:
As Rhiannon says, choose a well established boot sale. It doesn't have to be a weekly one (some of the best ones are only held a few times a year) but it should be big.
Obvious I know, but do not go if it is very cold or rainy. Customers will be thin on the ground and you will have a miserable time. Phone the met office weather call service the night before and just before you intend to leave in the morning. If a little rain only is forecast, be pessimistic and prepare for the worst, and bring waterproof sheeting, tarpaulin etc. There is no point in damaging your stock - it negates any profit you make.
Once you are there, it is likely regulars and dealers will pick you out at a newie. Lots of stalls are there week in week out. One-off stall-holders have the best bargains. Dealers will assume you just want to make a quick profit and sell everything in one go. Don't be surprised if they chat you up and ask if you're clearing your loft or emigrating etc etc. They are trying to find out how desperate you are to sell everything that day. So let it be known that you do boot sales elsewhere, and are in no hurry to sell.
If you see someone grab lots of items of the same sort - designer childrens clothes, old plates, second hand books etc - ask them if they collect this sort of stuff or if they sell it on. If they say 'yes' it's worth taking their phone number and contacting them the next time you have similar items to sell.
It's a good idea to limit haggling. It is very time consuming and if your attention is focussed for too long on one customer,you may miss making a better sale to another customer. On the other hand, people like to feel you are flexible, especially if they buy more than one item.
When I ran my stall, I used to have a notice that said, 'discount if you buy two or more items'. The discount was decided by me, and depended on the value of the items and how much I wanted to get rid of them. This really did encourage people to linger and buy more than one thing. What's more, I could stop hard haggling over a single item by saying they got a discount only if they bought two things. Lastly it's good to create a crowd at your stall. It attracts others.
For position, try and get a corner pitch. You get double the frontage. Don't be directed to a pitch that's too off the beaten track, though.
To make sure your pricing is right, one of you should take a wander round the boot fair and ask the price of similar items.
If a customer offers you a low-ish price for an item, you can always tell them to come back at the end of the morning, and talk to you then.
If someone asks you to reserve something, only do it for a fixed time.
If you are selling clothes, try and borrow a rail and hangers to display them. You get higher prices for them than if they are in a heap on the ground. Also, avoid cheap metal hangers otherwise you will be picking clothes off the ground all day.
Remember to take lots of change with you, because you can bet your bottom dollor that your first customers will hand you £20.00 notes.
Some of this may be a bit obvious, Kia, but I hope it helps!