Daniel Koster said:
Price is determined by demand. I have a knife here that I haven't been able to
give away....

.....there's no demand for it.
give? i'm all ears
my 2 cents... dan is ON da money... but i have some thoughts.
the item. have something to sell that probably someone wants. learn the rules of the forumboard/etc, some places have harsh rules (like links), and other such.
the client. invariably someone will contact you. scope them out. search on the forum. stand up guy? crook? good, bad, ugly might give you a hint. "dude is AAA++++ all the way" to "the dude forges money order, beware!".
pics. get a free hosting account, there's several. i like imageshack. free. really. take many good photos.
email. get a free email account just for selling - keep the spam and traffic local to that. i like gmail. it's free, it's good, it's searchable (the feds want to know all ;>)
communicate. BE POLITE but not a pushover. don't ignore emails and phone calls (WHAT? you gave them your phone? are you crazy?). talk to your client(s). assure them. be chatty if you want, but don't go nuts either... minimum basic: "i accept your offer", "your payment has cleared/arrived", "your package is shipped on this date, via this method, here's a conf #, should arrive in N to M days or so (prevents them from assuming it's instant shipping ;>)"... if they keep writing back within the shipping period, DO assure them, once, that it was shipping, and they should contact you in M+2 days or something...
description. write up a good solid description. be honest of any flaws. some people are very upfront and even offer a 5-7 day refund policy. others are "no refunds, no way"
money. goto a few shops and find out how much your item sells for. pick a price you want that's realistic. i always include shipping, fees, everything in the price. just name one price, too many complications messes up the deal. i do paypal. i'll take a [postal] money order but i'll wait 2-3 weeks. i do NOT take personal checks unless i've dealt with the person a bunch in some way, and that's the best way for them/me.
shipping. usually usps flat rate shipping, priority, very rarely express. ALWAYS delivery conf, insurance often extra if client wants it (or it's part of the price...) USPS provides free boxes, and for very heavy items that fit in the box, have cheap one price flat rates. i've never used fed ex for personal needs, but they're good too. ups sadly, cannot be trusted imho... too many stories and issues....
packing: free boxes if you go with usps... otherwise, use a good box. reinforce every single edge with GOOD tape. if you're paranoid, use anti-tamper tape over THAT. tape your labels down too. put a copy of the addresses inside the box. speaking of inside... knives are sharp. wrap that puppy well, take no chances it can de-sheath, or for gawds sake, if there is no sheath, wrap the blade with SOMETHing to keep it poking through, cutting itself out. this box WILL be dropped. put something over the tip and the edge. then wrap it again. plenty of bubble wrap or other packing materials. you should be able to shake your package and hear NOTHING move. really. i'm nutso about that. a customer should have to WORK to get their item

but really, i've never had anyway have an issue with something lost, or crunched, and i never want to.
feedback. there's a feedback mechanism usually on your forum/site of choice. use it. good/fantastic, bad/horrible. try to negotiate with the client first.
kaching!
trades go about the same way, just that you have to trust the other person... and you're good.
but mostly, have fun, can't think of much more...
bladite