After a seller agrees to ship to me internationally, the system sometimes refuses to snipe because I live in the wrong country. I lost several nice bargains!
To avoid this problem, I would like to log in as "a local buyer" Could this be a new option?
I would like to enter the item nr price, snipe time and country in the future

If the seller agreed, it's not enough. He/she will have to update their auctions to allow your bids.

The seller can either remove that restrictions or add your eBay user id to the exception list of allowed bidders.
But it must be done by the seller only.

I live in The Netherlands, my account is at ebay.com (not .nl)
I have no problems bidding manually in foreign countries
when I log in at my.ebay.co.uk , I can manually bid at any item that is for locals only.
But I do want to use Myibay doing the same thing and not ask the seller to change his settings just for me.

Your account is not assigned for some specific server, it will work both for ebay.com and ebay.nl (and others).

When the seller put a restrictions, then he/she can remove it only. I can not remove someone'e else restrictions from their auctions.
This restriction is usually set when the seller does not want bidders where he/she does not ship to bid on his/her items.
If seller agreed to ship it for you, then your eBay user id must be added to the exception list of allowed bidders. If not, your bid will be refused by eBay.

Myibay can not change or remove the restriction which was set by the seller.