Should really depend on what you are suing for, money ebay will fight tooth and nail, to get sellers to fulfil their obligations that they have quite clearly advertised on ebays site, should not be too difficult.
Ebay has a responsibility to both seller and buyer that the terms and conditions of any contract must be met.
But this leads to another question that undermines this whole site why cant ebay force a seller to meet his terms and conditions they are what the seller advertised.
Money is not everything in this world in fact its pretty useless, if a business like ebay cannot be trusted to make sure sellers honor the contract between buyer and seller then the site is undermined and nothing more than a cash grab fleecing the hard earned money of the buyer and giving no recourse to action on the buyers behalf.
60% of any business is reputation no reputation no business.
That a seller can renege on a guarantee impacts on the site directly so quite simply do not allow any guarantees at all or make the sellers honor the guarantees given.