The SDK was designed for use on the frontend side. It uses some libraries which usually are available in browser only.
To use the SDK on the server side (backend):

Install dependencies:

yarn add form-data
yarn add node-fetch

Add dependencies to global

global.FormData = require("form-data")
global.window = {
  fetch: require("node-fetch"),
  dispatchEvent: () => {
  },
}
global.CustomEvent = function CustomEvent() {
  return
}

You can check our backend example which buys Ethereum NFT on testnet

Pass private key, node RPC URL, network ID, item ID for buyout and start:

ETH_PRIVATE_KEY="0x..." \
ETHEREUM_RPC_URL="https://goerli.infura.io/..." \
ETHEREUM_NETWORK_ID="4" \
BUYOUT_ITEM_ID="0x1AF7A7555263F275433c6Bb0b8FdCD231F89B1D7:102581254137174039089845694331937600507918590364933200920056519678660477714440" \
ts-node packages/sdk-examples/src/backend/ethereum/buy.ts

Conclusion

Rarible SDK can be used in any javascript-enabled environment including backend. You have to provide implementations for form-data and fetch