I have some RP involved in buying and selling items, especially the more powerful things that make people nervous or that simply require a powerful caster.
I handle item sales on a variable time basis. Larger cities will have a more frequent check to reflect the odds that an interested buyer might wander into town or acquire funds they lacked before. Major cities get checks weekly, smaller towns monthly and thorps quarterly. If they get lucky, someone's already asked the guild for that particular item and it can be sold almost immediately. Other items may take a while to sell.
Most of the magic item market is handled by the mage's guilds (think Krynn's Towers of High Sorcery). The guild acts as a clearing house, connecting buyers with sellers while providing a degree of anonymity. The guild gets a cut (part of the markup) but takes a smaller bite from guild members on both the buying and the selling, making guild membership valuable (buy at 85%, sell at 75%)
The Churches (note the capital letter, this refers to the major organization not individual parishes) have their own magic item market but it tends towards more of a loan than a sale, with the exception of certain "public good' items (e.g. potions of curing). Items for sale are disposed of through the mages' guild who wisely treats all local clergy as guildmembers in return for divine favors. With a letter of introduction from the local priest, traveling clerics are extended those same privileges.
The party also has the option of selling their item to a dealer. Dealers are rare, primarily being merchant houses that have the resources to protect magic items and enough various agents to find buyers across a wide area. Dealers provide immediate cash but at a low value. (This is the 50% sale value)
There are two major festivals in the campaign's home region that involve auctions of valuable items, including magic. These have a variable pricing that's pretty much take-it-or-leave-it but are generally more profitable than the above process. Since one happens in spring and the other fall, it requires a long campaign timeline to make them worthwhile.
The party can also open a storefront, which requires some upfront fees to the locals and of course, maintaining security. The party has at times done so during the festivals to offload large quantities of common non-magical loot, such as the dozens of swords, shields and suits of armor one gathers from wiping out a bandit tribe.
Note that while I have the various sources for magical items available, the majority of times the party has to commission items from mages or clerics, going through the guild or the Churches.
I don't make it an all-day affair on purpose but at times the party tends to go a bit wild with the shopping after a major windfall and, if they are in need of maximum return, they will exert RP effort to achieve the greater cash return of the more complex process. By giving the party options of different methods of sale with the more RP-intensive ones being the greatest, if not fastest, return, it allows them to get as involved as they feel appropriate.
As far as the "time away from killing beasties" goes, my group has just as much fun arranging for a festival as they do a good slugfest. I don't force my players to have a particular play style; I'm flexible from hack'n slash to a mostly social campaign. They tend to choose the amount of combat they experience by deciding what to focus their time on. I occassionaly ambush them to remind them about their unresolved enemy problems but those ambushes tend to happen only after a couple of combat-free game sessions. Yes, I said "multiple combat-free game sessions." Completely at the players' whims. If they wanted combat, they have more than enough known enemies they could pursue.