I have an difficult time to come out the angle, always confuse about the angle when brainstorm the angle.*
For the example: Offer: aliexpress shopping (CPI)
Basically two things come out from my mind, first thing is do i need to only emphasize on what unique benefit/selling point that "aliexpress" gives us as angle? such as,
"Top shopping marketplace at your fingertips"
"Top shopping apps across the asia country"
"Most popular online shopping platform"
"Fuss-Free payment options make you feel no worry when make online purchase only in aliexpress "
"Shop whenever you are with aliexpress"
"Free delivery across china"
"Access 100+ million product in aliexpress"
Or second thing i just need to think the benefit/reason that usually online shopping gives us as angle?such as*
"Convenient"
"Cheap"
"Avoid the crowd"
"Variety product"
Another constraint that i face is when i create a 320x50 banner, i always got limitation to write the long headline, any suggestion?
Moreover, 40kb file size for .gif (320x250 banner) is it too big or need to compress somemore?
Please feel free to critique and comment on my stupid question to make me move forward.
Cheers.
I would focus on the benefit discount/ cheap. It all depends on where you are targeting. I have an idea for an angle.
1.) Get any product on Amazon 82% off here
2.) Amazon hates them. Everything's 83% off.
Something like that.
Then Niche it. Focus on different niches with the angles above, flashlight, dog collars, hats, etc
I've been running a rather big campaign for an e-shop selling goods related to "sleeping" - so mattresses, beds, pillows, blankets etc ... so it can be related to your example with aliexpress a little bit, thou they offer a mega amount of products compared to this one.
What worked the best was a combination of specific product line, for example pillow and the benefits they offered - low price, fast delivery, free shipping. So pretty much very similar to what mike35 already suggested.
It worked the best on adwords, the ads were like this : Quality Mattresses For Cheap : 24 hours delivery, free shipping and handling, money back guarantee. - its a translation, the original looked better in my native language, but you get the idea.
When buying goods, people want low prices, high quality, fast delivery and guarantees - focus on those.
1. Chinese prices, world level quality. Now with money back guarantee!
2. Quality goods for cheap? Yes, its possible. Fast shipping worldwide.
angles suggested by mike look pretty good too :
3. Still shopping on ebay? Try aliexpress and buy twice as much for the same $$$.
4. Amazon and Ebay are loosing market share to aliexpress! Find out why.
I could go on and on but Im sure you understand what Im trying to tell you 
I would focus on a specific thing aliexpress sells. Ie. Best place to buy baby clothes and target newlyweds, best place to buy knitting supplies and target people who knit, etc
Thanks to mike35,matuloo and johnaff for your valuable opinion about the shopping angles.. will try out few and test it..