Hello,
Unfortunately, I do not think this would be possible if the seller plans on keeping their install. There was a time with Sims 3 when only the disc version were available where people would install expansion packs onto one computer and then sell it so someone else could install it onto their computer - I think this was before EA had verification systems to prevent this.
However, I think almost all versions of Sims 3 are now tied to a specific Origin account holder, so I do not believe such transferring of second-hand expansion packs is possible any more. There is a small chance that if the seller deactivates the code for the expansion pack from their Origin account, it may be possible that you could install it.
I hope this helps and keep simming,
Cody
