I’m not having the problems in this thread at this moment besides maybe their search engine sucks.
I’m not defending them at all so don’t take this the wrong way but I will tell you my experiences.
.
I ran into the site wanting to do the captcha thing a few months back, I had trouble logging in but finally would get in.
Then all of a sudden I would add things to my cart and try to pay for them and it denied the sale. This went on for a week or so.
Then all of a sudden the transaction and payment would go thru and I thought it was fixed some how.
The next day all my orders were cancelled without any explantion. I messaged the sellers and tried to pay them thru paypal and ask what happened.
Only a few responded and they told me to contact CS as they had no control over it but some wanted me to send the money paypal without any paypal invoice.
I didn’t feel comfortable without having a invoice for proof of purchase (I ask for one) so I never responded back.
Over the coarse of a week or so, I tried to make purchases a few more times and they would go thru and the next day all were cancelled again. 
I gave up figuring it must be my bank card stopping the transaction. I din’t try to buy anything for more than a month.
A few days ago I got bored I guess, and tried to purchase a few things. Everything went thru again and the next day all items were shipped.
So now I can purchase things again. The bad part is I did nothing at all, I didn’t contact anyone or call my bank, it just started working again.
Makes me believe from other comments in this thread that Ali’s system is royally screwed up. Its working for me now without any problems or captcha crap.
I don’t know if everyones experience will go this way but it seems their system has problems and is slowly being fixed or it has something to do with they are trying to verify you are legit, I don’t know. I had purchased many things from them already over the coarse of almost two years and it just quit working for me but now its back.
Maybe take a break from Ali a little while and it will work again. Just thought I would throw in my two cents of experience with them. 