In the genteel world of luxury, companies long felt that the Web was no place for merchandising exclusive products. And there was a gentlemen’s agreement with department stores not to siphon sales by reaching out directly to wealthy customers. Then, in came the recession, and out went the niceties. Department stores slashed prices on $1,200 handbags, while luxury lines fretted about losing their exclusivity. Now, come September, marcjacobs.com is going retail, 10 years after most brands opened Web showrooms.