Some people don't have the skills or the time to make their cosplays.
That happened woth my first 2 cosplays. I didn't know how to sew well then so I bought them.
Once my cosplay fell completely apart I had to sew it back together.
No. Some people just don't have an interest in learning how to sew, or they just aren't good at it. The money is still going to someone who sews cosplays. I wouldn't ever buy a cosplay online because I like to make them myself, but before I had my sewing machine I had a local commissioner that went to the local con make my cosplays for me.
The only thing that bothers me is when people cosplay but do a really poor job on the costume. i.e using real hair that isn't even styled like the character's, poor quality of work on the outfit, being really freaking inaccurate, not getting the right shoes and just wearing tennis shoes [I literally saw someone cosplay as Grell from Kuroshitsuji and she just wore these ugly grey tennis shoes...]. I just get offended when people do this because cosplaying is a form of art...and it shouldn't be half-assed as badly as I've seen before. Stuff like that bothers me, but not when someone buys the cosplay online. My sister and I looked at Deathcom's (a photography group that is around here...I dunno how many cons they go to really) photos from the local con. I'm not even exaggerating when I say that we looked at over 1,000 pictures, and there were only 12 Half decent, to decent cosplays. Honestly, the best ones there were from Star Wars...
But again, to answer your question it is ok to buy a cosplay online 3nodding
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Of course not! Now if they tried to claim they made it, that's a different story...
I have no interest in sewing, but I have a friend do all the sewing and pattern-making for me while I figure out how things will go together and do the other work such as making belts and armour and styling wigs.
It's always better when you or someone you know makes the cosplay, or if you pay the price to have a freelancer make it for you (not one of those mass producing sites). The cosplay fits better, uses the right fabric and colours, and is more accurate, but nobody should be excluded from the hobby just because they don't have the time, money, or skill to make the cosplay themselves.
No, but sewing is one of the most useful skills to have. It's up there with cooking as nudity is illegal, therefore a necessity.
I think when you sew your own costume it turns out better a lot of time though, more detail. Unless they commissioned a seamstress to sew their cosplay when they bought it, unlike it being on ebay or such.