A "business" can be a nebulous thing, particularly where it consists of stock and, more importantly, the accumulated knowledge of the owner. In this case, I assume that what you have is, in essence, a stock of postcards, some of which may have additional interest to stamp collectors. If you want to keep an eBay
store going, you basically have two choices: develop the expertise in postcards to run it; or put the material out there and hope that the potentially interested find it. The former would take a long time, and would benefit from your having a genuine interest in the material. The latter would get the material of your hands, without necessarily realising the best price. An alternative would be to offer the material at auction - an auction house would give you a sense of likely value, and might offer you a cash price or a slot in a forthcoming sale. Good luck!