Good luck with the fair. They are a lovely way of meeting fellow crafters and finding out what people think of your products face to face. Check with the organisers what is provided, do you need table and chairs. Are there tea/coffee facilities (if not take a flask) and take plenty to nibble on but nothing that is too difficult to eat! Take a couple of jumpers/coats, even indoors or on a nice day it can get quite chilly standing around. If you are able to take something to make with you it sometimes generates interest if people can see you in action. Don’t forget to take a float with plenty of change.
As far as amount of stock is concerned its difficult to say - I always take as much as I can, if it stays in boxes theres no harm done but it would be a shame to run out but don’t overcrowd your table all at once. Display wise its a good idea to try and get things at different heights, upturned boxes covered with fabric are a good cheap starting point. Try and make sure your items are clearly priced either individually or with one label e.g.: Cards £2.50 each etc.
Don’t jump on your customers but don’t ignore them either, try and make eye contact and a polite good morning/afternoon for starters and if they start showing an interest point them in the direction of other alternatives you may have. With your personalised items I would take along samples and state (on a notice as well as verbally) that they can be personalised but remember you will have to include postage in the cost unless you can personalise them there and then. When you write down the address get them to check you have got it right.
Good luck and let us know how you get on.