New and occasional sellers have limits on the number and value of their listings.
EBay is even stricter when the seller is offering high priced electronic and computer things.
Since you have sold at least 10 items (going only by Feedback Left for Others) I suspect what you are seeing is a (very badly written) notice that you haves reached those limits.
Many newbies (and returnees) have a monthly limit of 10 items and/or $1000. At a guess, this is a paternalistic move to protect both buyers and sellers from errors and frauds.
I'd suggest contacting Customer Service and asking about what your limits are and how you can get them raised.
In addition to the phone clerks in Help and Safety at the bottom of this page you could try :
https://twitter.com/askebay
https://www.facebook.com/eBay/
both of which have a better reputation for giving useful and honest advice.