Don't know really it depends. In some affiliate programs it's illegal, to be sure check the agreement with your affiliate.
I participate in two affiliate programs--in one you're allowed to do what you're saying and in the other you aren't. Look at the rules for the specific program.
In CJ affiliate programs, they usually specify whether or not you can do this on the Advertiser Information Main Page. If it doesn't say anything about that, do not presume you can. Check with the company just to be sure.
I buy stuff through my own links all the time. I think most do allow it.
how do they know if you are the one buying the product or service?
Just beware of scams (like the Nigerian scam) where they want to purchase from you but you'll never see the money.
How do they know? Well, if you are ordering from the same computer that you use to work on your site, that will be obvious. If you have stuff shipped to the same address as the address you used to register for the program, that will be obvious. If the name on your credit card is the same as the name you used to register your site for the program... You get the idea. There are other ways, too.
As said above it varies from merchant to merchant, best bet is to check with the affilate manager and ask him/her what the regulations are for them.
Giving the option of "Purchase from your site and get a commission" and "Purchase from a competitor", most would opt with the former I believe.
Certainly from a merchant perspective I would actively promote self purchases from my affiliates, if you have 5000 odd affiliates at Christmas then that can be an awful lot of sales from a very loyal customer base!
The point of buying through your own referral link so you get the commission (or rebate on the product) is something an Affiliate Commission Exchange can take care of (have a quick search in google for one or sticky me). This way you won't be breaking any rules of the site you're purchasing from.
The Affiliate Commission Exchange will match your Purchase intention with someone elses affiliate link, and in return match someone elses purchase intention with one of your referral links. So everyone helps each other out and you get rebates (earn commission) whenever you purchase online.
If you purchase online and have affiliate links yourself, you would be crazy not to use an affiliate commission exchange.
let say a merchant disallows buying through one's own website, what if i buy stuffs for my friends or family using the same computer everytime, do they count in such purchase? customer name, shipping address, and credit card info to be used will be different every time.
To me it is really unreasonable for such restriction. Because when i buy service or product from a company, i consider myself to be a customer of that company instead of a publisher or an affiliate.
what i am saying is the role of a publisher and a customer is totally different and they should be separate all of time.
A even better question to ask is: "you mean i can't buy stuff or service from your company for being an affiliate?" Or
"you disqualify me as a customer for being a publisher of your company?" lolz :)
Many affiliates do make purchases through their own links mostly to make "test" purchases to see whether an advertiser is tracking properly.
I have done it several times myself and found that some advertisers are not giving proper credit/clicks.
I can tell you right now, Amazon doesn't allow affiliates to purchase items through their own sites. If you do purchase an item through one of your amazon links, it may or may not be immediately noticed. Before you get your Amazon payout, they will take out the comission you would've earned from your earnings.