This is how I learned how a Trade Blanket is run, your experience may vary.
The rules of "standard An Tir Trade blanket"
Everyone sits around an open space, usually a "blanket" or other ground covering. Sometimes it is invite only, sometimes you can only participate if you are there at the start. MOST An Tir trade-blankets are very open, traders come and go over the course of the night. Usually if you are late, pick a good spot, and ask permission to joint the trade blanket :) DON'T jump in right when it is your turn, that's rude.
Whoever is hosting will then explain the rules, any prohibited items, etc. A lot of trade-blankets TRY to concentrate on hand-made items and supplies, but often things quickly deteriorate.
Then it is Trader #1's turn. (often the host will start, or ask someone specific to start).
When it is YOUR turn, you choose an item that you want to trade, and place it in front of you on the blanket. You then describe the item, being as flowery (or silly, or serious) as you wish. A minimum description includes basic descriptions like what it is made of :) You can pass it around the blanket for people to see up close, offer samples if it is edible, or whatever.
You (Trader #1), now waits for everyone else to make an offer.
Making an offer:
The Trader directly to the left of Trader #1 now has a choice of two actions:
1. Make an offer
2. Pass
If they wish to pass, they simply state: "pass". If the Trader wishes to make an offer, then they bring out a thing, describe it, and place it on the blanket in front of them.
This proceeds around the circle, until everyone present has had a chance to make an offer on Trader #1's item.
NOTE! Trader #1 CANNOT accept any trade until EVERYONE has had a chance to make an offer!
So, back to Trader #1.
When everyone has had a chance to put their proposed trade on the blanket, Trader #1 has 3 choices. (you may wish to inspect the offers on the blanket before making a decision ;)
1. Accept a trade. You and the person with the thing you want shake hands, and trade things. DONE!
2. Withdraw. If nothing on the table is a thing that you want and that you feel is comparable enough to want to trade your thing for it, you can withdraw your item at this stage. Your turn is now DONE.
3. Ask for a sweetener.
Asking for a sweetener, how and why.
There are two reasons to ask for a sweetener.
First, there are two offers on the blanket that you can't decide between. Don't identify who they are, that'll show your hand :) Just state you can't decide, and that you need a sweetener to make your choice.
Second (and a little more socially dicy), is that the level of items out are not equal to what you think you can get for your trade item, and you want a higher "price". This is....not always polite ;) It can be, if done well, but, yeah.
HOW to ask for a sweetener:
When everyone's stuff is on the blanket, and it is your (Trader #1's) turn to decide, state "I want a sweetener" (POLITELY! Possibly with admiring hemming and hawing over many of the items present!).
WHAT happens then?
Everyone leaves the thing they had initially put out, and, proceeding around the circle in order, each other trader has the opportunity to add a small item (a sweetener, to make the deal "sweeter") to their proposed trade. So, each trader has two options at this point:
1. Hold. They do not add anything to their offer
2. Add a small item (often with flowery descriptions of why you should pick them, grin).
After each trader has had a chance to add to their proposed trade, it is now back to Trader #1. Trader #1 now has TWO choices:
1. Accept a trade. As above, shake hands, exchange things, your turn is done.
2. Withdraw your item, and your turn is done.
Now it is Trader #2's turn to put out something, and everyone offers trades to Trader #2!
Pointers:
DON'T show your hand ;) Keep your stuff in a box, so you can surprise people with really good trades for something you want. Keep something big-ticket in your box in case something big-ticket that you WANT comes up!
DO bring snacks and drinks to share. This is a social event, a time to nosh, whisper, try to drive bargains, and generally socialize with other craft-types :)
DO bring lots of little things, both as sweeteners, and to put out as trades for beginners. Don't want to scare them off, but don't really want to trade your best stuff for what they brought. Some of them will have TOTALLY awesome stuff next year, and just didn't understand what they should bring...
DON'T bring anything that isn't documentably medieval. No imported knives, no monkey-pod wood bowls from the thrift store, no polyester.
DO bring items that you made with your own hands, and supplies for the crafts of others. The rules for each Trade Blanket are unique, so check them before you come expecting trades. Many trade blankets do not allow trades for consumables (food and drink).
Items I have seen : (handmade) wooden bow, pickles, mead, soap, dyed wool yarn, buckles, arrow heads, belt fittings, brooches, bone needles, trim, belt, knife, iron torque, needle case, spoons, shoes, unspun yarn, bars of silver, handblown drinking glasses, ceramics, boxes, chairs, game boards, salve, drop spindles, fabric, dye stuff, amber, glass beads, fur, drinking horns,