I'm using the same PayPal merchant account on multiple websites. Currently I'm using old-style PayPal links (<a href="https://www.paypal.com/xclick/..."></a>) with IPN notification.
Now I'd like to start using the PayPal Express API. But at least for a while I would have the old links still online on some sites.
The switch to PayPal Express seems to be per account. Will the old links and IPN continue to work ?
when you implement PayPal express checkout that will not hinder your PayPal links on whatever site you have them on. You can have PayPal Express Checkout with non-hosted button code on the site at the same time; no problem, at least while you're done with your testing, etc.
PayPal Express Checkout can be used across multiple sites; however APIs are only one per account. What this means is that if you want the funds to go to separate accounts for each site then you'll need a new PayPal account per site as you'll have to use different APIs with Express Checkout. That is the UserName/Password/Signature.
However you will have one IPN listener field for each account; so if you don't mind getting pinged for each site using the same IPN listener then you're fine.
You can just create specific prefix # for each site with the products themselves. Like store # 1 = 001Product# | Store # 2 002Product | etc. This will help differentiate IPN pings for each store and help you catalog it better. This is if you decide to use the same APIs for Express Checkout with the same IPN of the account. If so, you will have the PayPal button purchases along with the Express Checkout purchases you all the same data IPN wise and all funds will roll into the same account.
I hope I hit your mark on your question, but let me know if you need additional clarity as I'm happy to help. Have a good one.